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Mybaits 源码解析 (八)----- 全网最详细,没有之一:结果集 ResultSet 自动映射成实体类对象(上篇)

程序员文章站 2023-10-29 11:42:34
上一篇文章我们已经将SQL发送到了数据库,并返回了ResultSet,接下来就是将结果集 ResultSet 自动映射成实体类对象。这样使用者就无需再手动操作结果集,并将数据填充到实体类对象中。这可大大降低开发的工作量,提高工作效率。 映射结果入口 我们来看看上次看源码的位置 public ......

上一篇文章我们已经将sql发送到了数据库,并返回了resultset,接下来就是将结果集 resultset 自动映射成实体类对象。这样使用者就无需再手动操作结果集,并将数据填充到实体类对象中。这可大大降低开发的工作量,提高工作效率。

映射结果入口

我们来看看上次看源码的位置

public <e> list<e> query(statement statement, resulthandler resulthandler) throws sqlexception {
    preparedstatement ps = (preparedstatement)statement;
    //执行数据库sql
    ps.execute();
    //进行resultset自动映射
    return this.resultsethandler.handleresultsets(ps);
}

结果集的处理入口方法是 handleresultsets

public list<object> handleresultsets(statement stmt) throws sqlexception {
    
    final list<object> multipleresults = new arraylist<object>();

    int resultsetcount = 0;
    //获取第一个resultset,通常只会有一个
    resultsetwrapper rsw = getfirstresultset(stmt);
    //从配置中读取对应的resultmap,通常也只会有一个,设置多个是通过逗号来分隔,我们平时有这样设置吗?
    list<resultmap> resultmaps = mappedstatement.getresultmaps();
    int resultmapcount = resultmaps.size();
    validateresultmapscount(rsw, resultmapcount);

    while (rsw != null && resultmapcount > resultsetcount) {
        resultmap resultmap = resultmaps.get(resultsetcount);
        // 处理结果集
        handleresultset(rsw, resultmap, multipleresults, null);
        rsw = getnextresultset(stmt);
        cleanupafterhandlingresultset();
        resultsetcount++;
    }

    // 以下逻辑均与多结果集有关,就不分析了,代码省略
    string[] resultsets = mappedstatement.getresultsets();
    if (resultsets != null) {...}

    return collapsesingleresultlist(multipleresults);
}

在实际运行过程中,通常情况下一个sql语句只返回一个结果集,对多个结果集的情况不做分析 。实际很少用到。继续看handleresultset方法

private void handleresultset(resultsetwrapper rsw, resultmap resultmap, list<object> multipleresults, resultmapping parentmapping) throws sqlexception {
    try {
        if (parentmapping != null) {
            handlerowvalues(rsw, resultmap, null, rowbounds.default, parentmapping);
        } else {
            if (resulthandler == null) {
                // 创建默认的结果处理器
                defaultresulthandler defaultresulthandler = new defaultresulthandler(objectfactory);
                // 处理结果集的行数据
                handlerowvalues(rsw, resultmap, defaultresulthandler, rowbounds, null);
                // 将结果加入multipleresults中
                multipleresults.add(defaultresulthandler.getresultlist());
            } else {
                handlerowvalues(rsw, resultmap, resulthandler, rowbounds, null);
            }
        }
    } finally {
        closeresultset(rsw.getresultset());
    }
}

通过handlerowvalues 映射resultset结果,最后映射的结果会在defaultresulthandler的resultlist集合中,最后将结果加入到multipleresults中就可以返回了,我们继续跟进handlerowvalues这个核心方法

public void handlerowvalues(resultsetwrapper rsw, resultmap resultmap, resulthandler<?> resulthandler,
        rowbounds rowbounds, resultmapping parentmapping) throws sqlexception {
    if (resultmap.hasnestedresultmaps()) {
        ensurenorowbounds();
        checkresulthandler();
        // 处理嵌套映射,关于嵌套映射我们下一篇文章单独分析
        handlerowvaluesfornestedresultmap(rsw, resultmap, resulthandler, rowbounds, parentmapping);
    } else {
        // 处理简单映射,本文先只分析简单映射
        handlerowvaluesforsimpleresultmap(rsw, resultmap, resulthandler, rowbounds, parentmapping);
    }
}

我们可以通过resultmap.hasnestedresultmaps()知道查询语句是否是嵌套查询,如果resultmap中包含<association> 和 <collection>且其select属性不为空,则为嵌套查询,大家可以看看我第三篇文章关于解析 resultmap 节点。本文先分析简单的映射

private void handlerowvaluesforsimpleresultmap(resultsetwrapper rsw, resultmap resultmap,
        resulthandler<?> resulthandler, rowbounds rowbounds, resultmapping parentmapping) throws sqlexception {

    defaultresultcontext<object> resultcontext = new defaultresultcontext<object>();
    // 根据 rowbounds 定位到指定行记录
    skiprows(rsw.getresultset(), rowbounds);
    // resultset是一个集合,很有可能我们查询的就是一个list,这就就每条数据遍历处理
    while (shouldprocessmorerows(resultcontext, rowbounds) && rsw.getresultset().next()) {
        resultmap discriminatedresultmap = resolvediscriminatedresultmap(rsw.getresultset(), resultmap, null);
        // 从 resultset 中获取结果
        object rowvalue = getrowvalue(rsw, discriminatedresultmap);
        // 存储结果到resulthandler的resultlist,最后resultlist加入multipleresults中返回
        storeobject(resulthandler, resultcontext, rowvalue, parentmapping, rsw.getresultset());
    }
}

我们查询的结果很有可能是一个集合,所以这里要遍历集合,每条结果单独进行映射,最后映射的结果加入到resulthandler的resultlist

mybatis 默认提供了 rowbounds 用于分页,从上面的代码中可以看出,这并非是一个高效的分页方式,是查出所有的数据,进行内存分页。除了使用 rowbounds,还可以使用一些第三方分页插件进行分页。我们后面文章来讲,我们来看关键代码getrowvalue,处理一行数据

private object getrowvalue(resultsetwrapper rsw, resultmap resultmap) throws sqlexception {
    // 这个map是用来存储延迟加载的bountsql的,我们下面来看
    final resultloadermap lazyloader = new resultloadermap();
 // 创建实体类对象,比如 employ 对象
    object rowvalue = createresultobject(rsw, resultmap, lazyloader, null);
    if (rowvalue != null && !hastypehandlerforresultobject(rsw, resultmap.gettype())) {
        final metaobject metaobject = configuration.newmetaobject(rowvalue);
        boolean foundvalues = this.useconstructormappings;
        
        if (shouldapplyautomaticmappings(resultmap, false)) {
            //自动映射,结果集中有的column,但resultmap中并没有配置  
            foundvalues = applyautomaticmappings(rsw, resultmap, metaobject, null) || foundvalues;
        }
      // 根据 <resultmap> 节点中配置的映射关系进行映射
        foundvalues = applypropertymappings(rsw, resultmap, metaobject, lazyloader, null) || foundvalues;
        foundvalues = lazyloader.size() > 0 || foundvalues;
        rowvalue = foundvalues || configuration.isreturninstanceforemptyrow() ? rowvalue : null;
    }
    return rowvalue;
}

重要的逻辑已经注释出来了。分别如下:

  1. 创建实体类对象
  2. 自动映射结果集中有的column,但resultmap中并没有配置

  3. 根据 <resultmap> 节点中配置的映射关系进行映射

创建实体类对象

我们想将查询结果映射成实体类对象,第一步当然是要创建实体类对象了,下面我们来看一下 mybatis 创建实体类对象的过程。

private object createresultobject(resultsetwrapper rsw, resultmap resultmap, resultloadermap lazyloader, string columnprefix) throws sqlexception {

    this.useconstructormappings = false;
    final list<class<?>> constructorargtypes = new arraylist<class<?>>();
    final list<object> constructorargs = new arraylist<object>();

    // 调用重载方法创建实体类对象
    object resultobject = createresultobject(rsw, resultmap, constructorargtypes, constructorargs, columnprefix);
    if (resultobject != null && !hastypehandlerforresultobject(rsw, resultmap.gettype())) {
        final list<resultmapping> propertymappings = resultmap.getpropertyresultmappings();
        for (resultmapping propertymapping : propertymappings) {
            // 如果开启了延迟加载,则为 resultobject 生成代理类,如果仅仅是配置的关联查询,没有开启延迟加载,是不会创建代理类
            if (propertymapping.getnestedqueryid() != null && propertymapping.islazy()) {
                /*
                 * 创建代理类,默认使用 javassist 框架生成代理类。
                 * 由于实体类通常不会实现接口,所以不能使用 jdk 动态代理 api 为实体类生成代理。
                 * 并且将lazyloader传进去了
                 */
                resultobject = configuration.getproxyfactory()
                    .createproxy(resultobject, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
                break;
            }
        }
    }
    this.useconstructormappings =
        resultobject != null && !constructorargtypes.isempty();
    return resultobject;
}

我们先来看 createresultobject 重载方法的逻辑

private object createresultobject(resultsetwrapper rsw, resultmap resultmap, list<class<?>> constructorargtypes, list<object> constructorargs, string columnprefix) throws sqlexception {

    final class<?> resulttype = resultmap.gettype();
    final metaclass metatype = metaclass.forclass(resulttype, reflectorfactory);
    final list<resultmapping> constructormappings = resultmap.getconstructorresultmappings();

    if (hastypehandlerforresultobject(rsw, resulttype)) {
        return createprimitiveresultobject(rsw, resultmap, columnprefix);
    } else if (!constructormappings.isempty()) {
        return createparameterizedresultobject(rsw, resulttype, constructormappings, constructorargtypes, constructorargs, columnprefix);
    } else if (resulttype.isinterface() || metatype.hasdefaultconstructor()) {
        // 通过 objectfactory 调用目标类的默认构造方法创建实例
        return objectfactory.create(resulttype);
    } else if (shouldapplyautomaticmappings(resultmap, false)) {
        return createbyconstructorsignature(rsw, resulttype, constructorargtypes, constructorargs, columnprefix);
    }
    throw new executorexception("do not know how to create an instance of " + resulttype);
}

一般情况下,mybatis 会通过 objectfactory 调用默认构造方法创建实体类对象。看看是如何创建的

public <t> t create(class<t> type, list<class<?>> constructorargtypes, list<object> constructorargs) {
    class<?> classtocreate = this.resolveinterface(type);
    return this.instantiateclass(classtocreate, constructorargtypes, constructorargs);
}

<t> t instantiateclass(class<t> type, list<class<?>> constructorargtypes, list<object> constructorargs) {
    try {
        constructor constructor;
        if (constructorargtypes != null && constructorargs != null) {
            constructor = type.getdeclaredconstructor((class[])constructorargtypes.toarray(new class[constructorargtypes.size()]));
            if (!constructor.isaccessible()) {
                constructor.setaccessible(true);
            }

            return constructor.newinstance(constructorargs.toarray(new object[constructorargs.size()]));
        } else {
            //通过反射获取构造器
            constructor = type.getdeclaredconstructor();
            if (!constructor.isaccessible()) {
                constructor.setaccessible(true);
            }
            //通过构造器来实例化对象
            return constructor.newinstance();
        }
    } catch (exception var9) {
        throw new reflectionexception("error instantiating " + type + " with invalid types (" + argtypes + ") or values (" + argvalues + "). cause: " + var9, var9);
    }
}

很简单,就是通过反射创建对象

结果集映射

映射结果集分为两种情况:一种是自动映射(结果集有但在resultmap里没有配置的字段),在实际应用中,都会使用自动映射,减少配置的工作。自动映射在mybatis中也是默认开启的。第二种是映射resultmap中配置的,我们分这两者映射来看

自动映射

private boolean applyautomaticmappings(resultsetwrapper rsw, resultmap resultmap, metaobject metaobject, string columnprefix) throws sqlexception {

    // 获取 unmappedcolumnautomapping 列表
    list<unmappedcolumnautomapping> automapping = createautomaticmappings(rsw, resultmap, metaobject, columnprefix);
    boolean foundvalues = false;
    if (!automapping.isempty()) {
        for (unmappedcolumnautomapping mapping : automapping) {
            // 通过 typehandler 从结果集中获取指定列的数据
            final object value = mapping.typehandler.getresult(rsw.getresultset(), mapping.column);
            if (value != null) {
                foundvalues = true;
            }
            if (value != null || (configuration.iscallsettersonnulls() && !mapping.primitive)) {
                // 通过元信息对象设置 value 到实体类对象的指定字段上
                metaobject.setvalue(mapping.property, value);
            }
        }
    }
    return foundvalues;
}

首先是获取 unmappedcolumnautomapping 集合,然后遍历该集合,并通过 typehandler 从结果集中获取数据,最后再将获取到的数据设置到实体类对象中。

unmappedcolumnautomapping 用于记录未配置在 <resultmap> 节点中的映射关系。它的代码如下:

private static class unmappedcolumnautomapping {

    private final string column;
    private final string property;
    private final typehandler<?> typehandler;
    private final boolean primitive;

    public unmappedcolumnautomapping(string column, string property, typehandler<?> typehandler, boolean primitive) {
        this.column = column;
        this.property = property;
        this.typehandler = typehandler;
        this.primitive = primitive;
    }
}

仅用于记录映射关系。下面看一下获取 unmappedcolumnautomapping 集合的过程,如下:

private list<unmappedcolumnautomapping> createautomaticmappings(resultsetwrapper rsw, resultmap resultmap, metaobject metaobject, string columnprefix) throws sqlexception {

    final string mapkey = resultmap.getid() + ":" + columnprefix;
    // 从缓存中获取 unmappedcolumnautomapping 列表
    list<unmappedcolumnautomapping> automapping = automappingscache.get(mapkey);
    // 缓存未命中
    if (automapping == null) {
        automapping = new arraylist<unmappedcolumnautomapping>();
        // 从 resultsetwrapper 中获取未配置在 <resultmap> 中的列名
        final list<string> unmappedcolumnnames = rsw.getunmappedcolumnnames(resultmap, columnprefix);
        for (string columnname : unmappedcolumnnames) {
            string propertyname = columnname;
            if (columnprefix != null && !columnprefix.isempty()) {
                if (columnname.touppercase(locale.english).startswith(columnprefix)) {
                    propertyname = columnname.substring(columnprefix.length());
                } else {
                    continue;
                }
            }
            // 将下划线形式的列名转成驼峰式,比如 author_name -> authorname
            final string property = metaobject.findproperty(propertyname, configuration.ismapunderscoretocamelcase());
            if (property != null && metaobject.hassetter(property)) {
                // 检测当前属性是否存在于 resultmap 中
                if (resultmap.getmappedproperties().contains(property)) {
                    continue;
                }
                // 获取属性对于的类型
                final class<?> propertytype = metaobject.getsettertype(property);
                if (typehandlerregistry.hastypehandler(propertytype, rsw.getjdbctype(columnname))) {
                    final typehandler<?> typehandler = rsw.gettypehandler(propertytype, columnname);
                    // 封装上面获取到的信息到 unmappedcolumnautomapping 对象中
                    automapping.add(new unmappedcolumnautomapping(columnname, property, typehandler, propertytype.isprimitive()));
                } else {
                    configuration.getautomappingunknowncolumnbehavior()
                        .doaction(mappedstatement, columnname, property, propertytype);
                }
            } else {
                configuration.getautomappingunknowncolumnbehavior()
                    .doaction(mappedstatement, columnname, (property != null) ? property : propertyname, null);
            }
        }
        // 写入缓存
        automappingscache.put(mapkey, automapping);
    }
    return automapping;
}

先来看看从 resultsetwrapper 中获取未配置在 <resultmap> 中的列名

public list<string> getunmappedcolumnnames(resultmap resultmap, string columnprefix) throws sqlexception {
    list<string> unmappedcolumnnames = unmappedcolumnnamesmap.get(getmapkey(resultmap, columnprefix));
    if (unmappedcolumnnames == null) {
        // 加载已映射与未映射列名
        loadmappedandunmappedcolumnnames(resultmap, columnprefix);
        // 获取未映射列名
        unmappedcolumnnames = unmappedcolumnnamesmap.get(getmapkey(resultmap, columnprefix));
    }
    return unmappedcolumnnames;
}

private void loadmappedandunmappedcolumnnames(resultmap resultmap, string columnprefix) throws sqlexception {
    list<string> mappedcolumnnames = new arraylist<string>();
    list<string> unmappedcolumnnames = new arraylist<string>();
    final string uppercolumnprefix = columnprefix == null ? null : columnprefix.touppercase(locale.english);
    // 获取 <resultmap> 中配置的所有列名
    final set<string> mappedcolumns = prependprefixes(resultmap.getmappedcolumns(), uppercolumnprefix);
    /*
     * 遍历 columnnames,columnnames 是 resultsetwrapper 的成员变量,保存了当前结果集中的所有列名
     * 这里是通过resultset中的所有列名来获取没有在resultmap中配置的列名
     * 意思是后面进行自动赋值时,只赋值查出来的列名
     */
    for (string columnname : columnnames) {
        final string uppercolumnname = columnname.touppercase(locale.english);
        // 检测已映射列名集合中是否包含当前列名
        if (mappedcolumns.contains(uppercolumnname)) {
            mappedcolumnnames.add(uppercolumnname);
        } else {
            // 将列名存入 unmappedcolumnnames 中
            unmappedcolumnnames.add(columnname);
        }
    }
    // 缓存列名集合
    mappedcolumnnamesmap.put(getmapkey(resultmap, columnprefix), mappedcolumnnames);
    unmappedcolumnnamesmap.put(getmapkey(resultmap, columnprefix), unmappedcolumnnames);
}

首先是从当前数据集中获取列名集合,然后获取 <resultmap> 中配置的列名集合。之后遍历数据集中的列名集合,并判断列名是否被配置在了 <resultmap> 节点中。若配置了,则表明该列名已有映射关系,此时该列名存入 mappedcolumnnames 中。若未配置,则表明列名未与实体类的某个字段形成映射关系,此时该列名存入 unmappedcolumnnames 中。

映射result节点

接下来分析一下 mybatis 是如何将结果集中的数据填充到已配置resultmap映射的实体类字段中的。

private boolean applypropertymappings(resultsetwrapper rsw, resultmap resultmap, metaobject metaobject,resultloadermap lazyloader, string columnprefix) throws sqlexception {
    
    // 获取已映射的列名
    final list<string> mappedcolumnnames = rsw.getmappedcolumnnames(resultmap, columnprefix);
    boolean foundvalues = false;
    // 获取 resultmapping集合
    final list<resultmapping> propertymappings = resultmap.getpropertyresultmappings();
    // 所有的resultmapping遍历进行映射
    for (resultmapping propertymapping : propertymappings) {
        string column = prependprefix(propertymapping.getcolumn(), columnprefix);
        if (propertymapping.getnestedresultmapid() != null) {
            column = null;
        }
        if (propertymapping.iscompositeresult()
            || (column != null && mappedcolumnnames.contains(column.touppercase(locale.english)))
            || propertymapping.getresultset() != null) {
            
            // 从结果集中获取指定列的数据
            object value = getpropertymappingvalue(rsw.getresultset(), metaobject, propertymapping, lazyloader, columnprefix);
            
            final string property = propertymapping.getproperty();
            if (property == null) {
                continue;

            // 若获取到的值为 defered,则延迟加载该值
            } else if (value == defered) {
                foundvalues = true;
                continue;
            }
            if (value != null) {
                foundvalues = true;
            }
            if (value != null || (configuration.iscallsettersonnulls() && !metaobject.getsettertype(property).isprimitive())) {
                // 将获取到的值设置到实体类对象中
                metaobject.setvalue(property, value);
            }
        }
    }
    return foundvalues;
}

private object getpropertymappingvalue(resultset rs, metaobject metaresultobject, resultmapping propertymapping,resultloadermap lazyloader, string columnprefix) throws sqlexception {

    if (propertymapping.getnestedqueryid() != null) {
        // 获取关联查询结果
        return getnestedquerymappingvalue(rs, metaresultobject, propertymapping, lazyloader, columnprefix);
    } else if (propertymapping.getresultset() != null) {
        addpendingchildrelation(rs, metaresultobject, propertymapping);
        return defered;
    } else {
        final typehandler<?> typehandler = propertymapping.gettypehandler();
        final string column = prependprefix(propertymapping.getcolumn(), columnprefix);
        // 从 resultset 中获取指定列的值
        return typehandler.getresult(rs, column);
    }
}

从 resultmap 获取映射对象 resultmapping 集合。然后遍历 resultmapping 集合,再此过程中调用 getpropertymappingvalue 获取指定指定列的数据,最后将获取到的数据设置到实体类对象中。

这里和自动映射有一点不同,自动映射是从直接从resultset 中获取指定列的值,但是通过resultmap多了一种情况,那就是关联查询,也可以说是延迟查询,此关联查询如果没有配置延迟加载,那么就要获取关联查询的值,如果配置了延迟加载,则返回defered

关联查询与延迟加载

我们的查询经常会碰到一对一,一对多的情况,通常我们可以用一条 sql 进行多表查询完成任务。当然我们也可以使用关联查询,将一条 sql 拆成两条去完成查询任务。mybatis 提供了两个标签用于支持一对一和一对多的使用场景,分别是 <association> 和 <collection>。下面我来演示一下如何使用 <association> 完成一对一的关联查询。先来看看实体类的定义:

/** 作者类 */
public class author {
    private integer id;
    private string name;
    private integer age;
    private integer sex;
    private string email;
    
    // 省略 getter/setter
}

/** 文章类 */
public class article {
    private integer id;
    private string title;
    // 一对一关系
    private author author;
    private string content;
    private date createtime;
    
    // 省略 getter/setter
}

接下来看一下 mapper 接口与映射文件的定义。

public interface articledao {
    article findone(@param("id") int id);
    author findauthor(@param("id") int authorid);
}

 

<mapper namespace="xyz.coolblog.dao.articledao">
    <resultmap id="articleresult" type="article">
        <result property="createtime" column="create_time"/>
        //column 属性值仅包含列信息,参数类型为 author_id 列对应的类型,这里为 integer
        //意思是将author_id做为参数传给关联的查询语句findauthor
        <association property="author" column="author_id" javatype="author" select="findauthor"/>
    </resultmap>

    <select id="findone" resultmap="articleresult">
        select
            id, author_id, title, content, create_time
        from
            article
        where
            id = #{id}
    </select>

    <select id="findauthor" resulttype="author">
        select
            id, name, age, sex, email
        from
            author
        where
            id = #{id}
    </select>
</mapper>

开启延迟加载

<!-- 开启延迟加载 -->
<setting name="lazyloadingenabled" value="true"/>
<!-- 关闭积极的加载策略 -->
<setting name="aggressivelazyloading" value="false"/>
<!-- 延迟加载的触发方法 -->
<setting name="lazyloadtriggermethods" value="equals,hashcode"/>

此时association节点使用了select指向另外一个查询语句,并且将 author_id作为参数传给关联查询的语句

此时如果不开启延迟加载,那么会生成两条sql,先执行findone,然后通过findone的返回结果做为参数,执行findauthor语句,并将结果设置到author属性

如果开启了延迟加载呢?那么只会执行findone一条sql,当调用article.getauthor()方法时,才会去执行findauthor进行查询,我们下面来看看是如何实现的

我们还是要从上面映射result节点说起

private object getpropertymappingvalue(resultset rs, metaobject metaresultobject, resultmapping propertymapping,resultloadermap lazyloader, string columnprefix) throws sqlexception {

    if (propertymapping.getnestedqueryid() != null) {
        // 获取关联查询结果
        return getnestedquerymappingvalue(rs, metaresultobject, propertymapping, lazyloader, columnprefix);
    } else if (propertymapping.getresultset() != null) {
        addpendingchildrelation(rs, metaresultobject, propertymapping);
        return defered;
    } else {
        final typehandler<?> typehandler = propertymapping.gettypehandler();
        final string column = prependprefix(propertymapping.getcolumn(), columnprefix);
        // 从 resultset 中获取指定列的值
        return typehandler.getresult(rs, column);
    }
}

我们看到,如果resultmapping设置了关联查询,也就是association或者collection配置了select,那么就要通过关联语句来查询结果,并设置到实体类对象的属性中了。如果没配置select,那就简单,直接从resultset中通过列名获取结果。那我们来看看getnestedquerymappingvalue

private object getnestedquerymappingvalue(resultset rs, metaobject metaresultobject, resultmapping propertymapping, resultloadermap lazyloader, string columnprefix) throws sqlexception {

    // 获取关联查询 id,id = 命名空间 + <association> 的 select 属性值
    final string nestedqueryid = propertymapping.getnestedqueryid();
    final string property = propertymapping.getproperty();
    // 根据 nestedqueryid 获取关联的 mappedstatement
    final mappedstatement nestedquery = configuration.getmappedstatement(nestedqueryid);
    //获取关联查询mappedstatement的参数类型
    final class<?> nestedqueryparametertype = nestedquery.getparametermap().gettype();
    /*
     * 生成关联查询语句参数对象,参数类型可能是一些包装类,map 或是自定义的实体类,
     * 具体类型取决于配置信息。以上面的例子为基础,下面分析不同配置对参数类型的影响:
     *   1. <association column="author_id"> 
     *      column 属性值仅包含列信息,参数类型为 author_id 列对应的类型,这里为 integer
     * 
     *   2. <association column="{id=author_id, name=title}"> 
     *      column 属性值包含了属性名与列名的复合信息,mybatis 会根据列名从 resultset 中
     *      获取列数据,并将列数据设置到实体类对象的指定属性中,比如:
     *          author{id=1, name="陈浩"}
     *      或是以键值对 <属性, 列数据> 的形式,将两者存入 map 中。比如:
     *          {"id": 1, "name": "陈浩"}
     *
     *      至于参数类型到底为实体类还是 map,取决于关联查询语句的配置信息。比如:
     *          <select id="findauthor">  ->  参数类型为 map
     *          <select id="findauthor" parametertype="author"> -> 参数类型为实体类
     */
    final object nestedqueryparameterobject = prepareparameterfornestedquery(rs, propertymapping, nestedqueryparametertype, columnprefix);
    object value = null;
    if (nestedqueryparameterobject != null) {
        // 获取 boundsql,这里设置了运行时参数,所以这里是能直接执行的
        final boundsql nestedboundsql = nestedquery.getboundsql(nestedqueryparameterobject);
        final cachekey key = executor.createcachekey(nestedquery, nestedqueryparameterobject, rowbounds.default, nestedboundsql);
        final class<?> targettype = propertymapping.getjavatype();

        if (executor.iscached(nestedquery, key)) {
            executor.deferload(nestedquery, metaresultobject, property, key, targettype);
            value = defered;
        } else {
            // 创建结果加载器
            final resultloader resultloader = new resultloader(configuration, executor, nestedquery, nestedqueryparameterobject, targettype, key, nestedboundsql);
            // 检测当前属性是否需要延迟加载
            if (propertymapping.islazy()) {
                // 添加延迟加载相关的对象到 loadermap 集合中
                lazyloader.addloader(property, metaresultobject, resultloader);
                value = defered;
            } else {
                // 直接执行关联查询
                // 如果只是配置关联查询,但是没有开启懒加载,则直接执行关联查询,并返回结果,设置到实体类对象的属性中
                value = resultloader.loadresult();
            }
        }
    }
    return value;
}

下面先来总结一下该方法的逻辑:

  1. 根据 nestedqueryid 获取 mappedstatement
  2. 生成参数对象
  3. 获取 boundsql
  4. 创建结果加载器 resultloader
  5. 检测当前属性是否需要进行延迟加载,若需要,则添加延迟加载相关的对象到 loadermap 集合中
  6. 如不需要延迟加载,则直接通过结果加载器加载结果

以上流程中针对一级缓存的检查是十分有必要的,若缓存命中,可直接取用结果,无需再在执行关联查询 sql。若缓存未命中,接下来就要按部就班执行延迟加载相关逻辑

我们来看一下添加延迟加载相关对象到 loadermap 集合中的逻辑,如下:

public void addloader(string property, metaobject metaresultobject, resultloader resultloader) {
    // 将属性名转为大写
    string upperfirst = getuppercasefirstproperty(property);
    if (!upperfirst.equalsignorecase(property) && loadermap.containskey(upperfirst)) {
        throw new executorexception("nested lazy loaded result property '" + property +
                                    "' for query id '" + resultloader.mappedstatement.getid() +
                                    " already exists in the result map. the leftmost property of all lazy loaded properties must be unique within a result map.");
    }
    // 创建 loadpair,并将 <大写属性名,loadpair对象> 键值对添加到 loadermap 中
    loadermap.put(upperfirst, new loadpair(property, metaresultobject, resultloader));
}

我们再来回顾一下文章开始的创建实体类

private object createresultobject(resultsetwrapper rsw, resultmap resultmap, resultloadermap lazyloader, string columnprefix) throws sqlexception {

    this.useconstructormappings = false;
    final list<class<?>> constructorargtypes = new arraylist<class<?>>();
    final list<object> constructorargs = new arraylist<object>();

    // 调用重载方法创建实体类对象
    object resultobject = createresultobject(rsw, resultmap, constructorargtypes, constructorargs, columnprefix);
    if (resultobject != null && !hastypehandlerforresultobject(rsw, resultmap.gettype())) {
        final list<resultmapping> propertymappings = resultmap.getpropertyresultmappings();
        for (resultmapping propertymapping : propertymappings) {
            // 如果开启了延迟加载,则为 resultobject 生成代理类,如果仅仅是配置的关联查询,没有开启延迟加载,是不会创建代理类
            if (propertymapping.getnestedqueryid() != null && propertymapping.islazy()) {
                /*
                 * 创建代理类,默认使用 javassist 框架生成代理类。
                 * 由于实体类通常不会实现接口,所以不能使用 jdk 动态代理 api 为实体类生成代理。
                 * 并且将lazyloader传进去了
                 */
                resultobject = configuration.getproxyfactory()
                    .createproxy(resultobject, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
                break;
            }
        }
    }
    this.useconstructormappings =
        resultobject != null && !constructorargtypes.isempty();
    return resultobject;
}

如果开启了延迟加载,并且有关联查询,此时是要创建一个代理对象的,将上面存放bondsql的lazyloader和创建的目标对象resultobject 作为参数传进去。

mybatis提供了两个实现类cglibproxyfactory和javassistproxyfactory,分别基于org.javassist:javassist和cglib:cglib进行实现。createproxy方法就是实现懒加载逻辑的核心方法,也是我们分析的目标。

cglibproxyfactory

cglibproxyfactory基于cglib动态代理模式,通过继承父类的方式生成动态代理类。

@override
public object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
  return enhancedresultobjectproxyimpl.createproxy(target, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
}

public static object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
  final class<?> type = target.getclass();
  enhancedresultobjectproxyimpl callback = new enhancedresultobjectproxyimpl(type, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
  //由cglibproxyfactory生成对象
  object enhanced = crateproxy(type, callback, constructorargtypes, constructorargs);
  //复制属性
  propertycopier.copybeanproperties(type, target, enhanced);
  return enhanced;
}

static object crateproxy(class<?> type, callback callback, list<class<?>> constructorargtypes, list<object> constructorargs) {
  enhancer enhancer = new enhancer();
  enhancer.setcallback(callback);
  //设置父类对象
  enhancer.setsuperclass(type);
  try {
    type.getdeclaredmethod(write_replace_method);
    // objectoutputstream will call writereplace of objects returned by writereplace
    if (log.isdebugenabled()) {
      log.debug(write_replace_method + " method was found on bean " + type + ", make sure it returns this");
    }
  } catch (nosuchmethodexception e) {
    enhancer.setinterfaces(new class[]{writereplaceinterface.class});
  } catch (securityexception e) {
    // nothing to do here
  }
  object enhanced;
  if (constructorargtypes.isempty()) {
    enhanced = enhancer.create();
  } else {
    class<?>[] typesarray = constructorargtypes.toarray(new class[constructorargtypes.size()]);
    object[] valuesarray = constructorargs.toarray(new object[constructorargs.size()]);
    enhanced = enhancer.create(typesarray, valuesarray);
  }
  return enhanced;
}

 可以看到,初始化enhancer,并调用构造方法,生成对象。从enhancer.setsuperclass(type);也能看出cglib采用的是继承父类的方式。

enhancedresultobjectproxyimpl

enhancedresultobjectproxyimpl实现了methodinterceptor接口,此接口是cglib拦截目标对象方法的入口,对目标对象方法的调用都会通过此接口的intercept的方法。

@override
public object intercept(object enhanced, method method, object[] args, methodproxy methodproxy) throws throwable {
  final string methodname = method.getname();
  try {
    synchronized (lazyloader) {
      if (write_replace_method.equals(methodname)) {
        object original;
        if (constructorargtypes.isempty()) {
          original = objectfactory.create(type);
        } else {
          original = objectfactory.create(type, constructorargtypes, constructorargs);
        }
        propertycopier.copybeanproperties(type, enhanced, original);
        if (lazyloader.size() > 0) {
          return new cglibserialstateholder(original, lazyloader.getproperties(), objectfactory, constructorargtypes, constructorargs);
        } else {
          return original;
        }
      } else {
        if (lazyloader.size() > 0 && !finalize_method.equals(methodname)) {
        /*
         * 如果 aggressive 为 true,或触发方法(比如 equals,hashcode 等)被调用,
         * 则加载所有的所有延迟加载的数据
         */
          if (aggressive || lazyloadtriggermethods.contains(methodname)) {
            lazyloader.loadall();
          } else if (propertynamer.issetter(methodname)) {
            // 如果使用者显示调用了 setter 方法,则将相应的延迟加载类从 loadermap 中移除
            final string property = propertynamer.methodtoproperty(methodname);
            lazyloader.remove(property);
          // 检测使用者是否调用 getter 方法
          } else if (propertynamer.isgetter(methodname)) {
            final string property = propertynamer.methodtoproperty(methodname);
            if (lazyloader.hasloader(property)) {
              // 执行延迟加载逻辑
              lazyloader.load(property);
            }
          }
        }
      }
    }
    //执行原方法(即父类方法)
    return methodproxy.invokesuper(enhanced, args);
  } catch (throwable t) {
    throw exceptionutil.unwrapthrowable(t);
  }
}

完整的代码

import java.lang.reflect.method;
import java.util.list;
import java.util.map;
import java.util.properties;
import java.util.set;

import net.sf.cglib.proxy.callback;
import net.sf.cglib.proxy.enhancer;
import net.sf.cglib.proxy.methodinterceptor;
import net.sf.cglib.proxy.methodproxy;

import org.apache.ibatis.executor.loader.abstractenhanceddeserializationproxy;
import org.apache.ibatis.executor.loader.abstractserialstateholder;
import org.apache.ibatis.executor.loader.proxyfactory;
import org.apache.ibatis.executor.loader.resultloadermap;
import org.apache.ibatis.executor.loader.writereplaceinterface;
import org.apache.ibatis.io.resources;
import org.apache.ibatis.logging.log;
import org.apache.ibatis.logging.logfactory;
import org.apache.ibatis.reflection.exceptionutil;
import org.apache.ibatis.reflection.factory.objectfactory;
import org.apache.ibatis.reflection.property.propertycopier;
import org.apache.ibatis.reflection.property.propertynamer;
import org.apache.ibatis.session.configuration;

/**
 * cglib代理工厂类,实现延迟加载属性
 * @author clinton begin
 */
public class cglibproxyfactory implements proxyfactory {

  /**
   * finalize方法
   */
  private static final string finalize_method = "finalize";
  /**
   * writereplace方法
   */
  private static final string write_replace_method = "writereplace";

  /**
   * 加载enhancer,这个是cglib的入口
   */
  public cglibproxyfactory() {
    try {
      resources.classforname("net.sf.cglib.proxy.enhancer");
    } catch (throwable e) {
      throw new illegalstateexception("cannot enable lazy loading because cglib is not available. add cglib to your classpath.", e);
    }
  }

  /**
   * 创建代理对象
   * @param target 目标对象
   * @param lazyloader 延迟加载器
   * @param configuration 配置类
   * @param objectfactory 对象工厂
   * @param constructorargtypes 构造函数类型[]
   * @param constructorargs  构造函数的值[]
   * @return
   */
  @override
  public object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
    return enhancedresultobjectproxyimpl.createproxy(target, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
  }

  /**
   * 创建一个反序列化代理
   * @param target 目标
   * @param unloadedproperties
   * @param objectfactory 对象工厂
   * @param constructorargtypes 构造函数类型数组
   * @param constructorargs 构造函数值
   * @return
   */
  public object createdeserializationproxy(object target, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
    return enhanceddeserializationproxyimpl.createproxy(target, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
  }

  @override
  public void setproperties(properties properties) {
      // not implemented
  }

  /**
   * 返回代理对象, 这个代理对象在调用任何方法都会调用本类的intercept方法
   * enhancer 认为这个就是自定义类的工厂,比如这个类需要实现什么接口
   * @param type 目标类型
   * @param callback 结果对象代理实现类,当中有invoke回调方法
   * @param constructorargtypes 构造函数类型数组
   * @param constructorargs 构造函数对应字段的值数组
   * @return
   */
  static object crateproxy(class<?> type, callback callback, list<class<?>> constructorargtypes, list<object> constructorargs) {
    // enhancer 配置调节代理对象的一些参数
    // 设置回调方法
    // 设置超类
    //判断当传入目标类型是否有writereplace方法,没有则配置一个有writereplace方法的接口(序列化写出)
    enhancer enhancer = new enhancer();
    enhancer.setcallback(callback);
    enhancer.setsuperclass(type);
    try {
      type.getdeclaredmethod(write_replace_method);
      // objectoutputstream will call writereplace of objects returned by writereplace
      if (logholder.log.isdebugenabled()) {
        logholder.log.debug(write_replace_method + " method was found on bean " + type + ", make sure it returns this");
      }
    } catch (nosuchmethodexception e) {
      //这个enhancer增加一个writereplaceinterface接口
      enhancer.setinterfaces(new class[]{writereplaceinterface.class});
    } catch (securityexception e) {
      // nothing to do here
    }
    //根据构造函数创建一个对象
    //无参构造
    //有参构造
    object enhanced;
    if (constructorargtypes.isempty()) {
      enhanced = enhancer.create();
    } else {
      class<?>[] typesarray = constructorargtypes.toarray(new class[constructorargtypes.size()]);
      object[] valuesarray = constructorargs.toarray(new object[constructorargs.size()]);
      enhanced = enhancer.create(typesarray, valuesarray);
    }
    return enhanced;
  }

  /**
   * 结果对象代理实现类,
   * 它实现方法拦截器的intercept方法
   */
  private static class enhancedresultobjectproxyimpl implements methodinterceptor {

    private final class<?> type;
    private final resultloadermap lazyloader;
    private final boolean aggressive;
    private final set<string> lazyloadtriggermethods;
    private final objectfactory objectfactory;
    private final list<class<?>> constructorargtypes;
    private final list<object> constructorargs;

    /**
     * 代理对象创建
     * @param type 目标class类型
     * @param lazyloader 延迟加载器
     * @param configuration 配置信息
     * @param objectfactory 对象工厂
     * @param constructorargtypes 构造函数类型数组
     * @param constructorargs 构造函数值数组
     */
    private enhancedresultobjectproxyimpl(class<?> type, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
      this.type = type;
      this.lazyloader = lazyloader;
      this.aggressive = configuration.isaggressivelazyloading();
      this.lazyloadtriggermethods = configuration.getlazyloadtriggermethods();
      this.objectfactory = objectfactory;
      this.constructorargtypes = constructorargtypes;
      this.constructorargs = constructorargs;
    }

    /**
     * 创建代理对象, 将源对象值赋值给代理对象
     * @param target 目标对象
     * @param lazyloader 延迟加载器
     * @param configuration 配置对象
     * @param objectfactory 对象工厂
     * @param constructorargtypes 构造函数类型数组
     * @param constructorargs 构造函数值数组
     * @return
     */
    public static object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
      //获取目标的类型
      //创建一个结果对象代理实现类(它实现cglib的methodinterface接口,完成回调作用invoke方法)
      final class<?> type = target.getclass();
      enhancedresultobjectproxyimpl callback = new enhancedresultobjectproxyimpl(type, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
      object enhanced = crateproxy(type, callback, constructorargtypes, constructorargs);
      propertycopier.copybeanproperties(type, target, enhanced);
      return enhanced;
    }

    /**
     * 回调方法
     * @param enhanced 代理对象
     * @param method 方法
     * @param args 方法参数
     * @param methodproxy 代理方法
     * @return
     * @throws throwable
     */
    @override
    public object intercept(object enhanced, method method, object[] args, methodproxy methodproxy) throws throwable {
      //获取方法名
      final string methodname = method.getname();
      try {
        // 同步获取延迟加载对象
        // 如果是执行writereplace方法(序列化写出)
        // 实例化一个目标对象的实例
        synchronized (lazyloader) {
          if (write_replace_method.equals(methodname)) {
            object original;
            if (constructorargtypes.isempty()) {
              original = objectfactory.create(type);
            } else {
              original = objectfactory.create(type, constructorargtypes, constructorargs);
            }
            // 将enhanced中的属性复制到orignal对象中
            // 如果延迟加载数量>0,
            propertycopier.copybeanproperties(type, enhanced, original);
            if (lazyloader.size() > 0) {
              return new cglibserialstateholder(original, lazyloader.getproperties(), objectfactory, constructorargtypes, constructorargs);
            } else {
              return original;
            }
          } else {
            //不是writereplace方法
            // 延迟加载长度大于0, 且不是finalize方法
            // configuration配置延迟加载参数,延迟加载触发的方法包含这个方法
            // 延迟加载所有数据
            if (lazyloader.size() > 0 && !finalize_method.equals(methodname)) {
              if (aggressive || lazyloadtriggermethods.contains(methodname)) {
                lazyloader.loadall();
                // setter方法,直接移除
              } else if (propertynamer.issetter(methodname)) {
                final string property = propertynamer.methodtoproperty(methodname);
                lazyloader.remove(property);
                // getter方法, 加载该属性
              } else if (propertynamer.isgetter(methodname)) {
                final string property = propertynamer.methodtoproperty(methodname);
                if (lazyloader.hasloader(property)) {
                  lazyloader.load(property);
                }
              }
            }
          }
        }
        return methodproxy.invokesuper(enhanced, args);
      } catch (throwable t) {
        throw exceptionutil.unwrapthrowable(t);
      }
    }
  }

  /**
   * 他继承抽象反序列化代理和实现了方法拦截
   */
  private static class enhanceddeserializationproxyimpl extends abstractenhanceddeserializationproxy implements methodinterceptor {

    private enhanceddeserializationproxyimpl(class<?> type, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      super(type, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
    }

    /**
     * 创建代理对象
     * @param target
     * @param unloadedproperties
     * @param objectfactory
     * @param constructorargtypes
     * @param constructorargs
     * @return
     */
    public static object createproxy(object target, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      final class<?> type = target.getclass();
      enhanceddeserializationproxyimpl callback = new enhanceddeserializationproxyimpl(type, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
      object enhanced = crateproxy(type, callback, constructorargtypes, constructorargs);
      propertycopier.copybeanproperties(type, target, enhanced);
      return enhanced;
    }

    @override
    public object intercept(object enhanced, method method, object[] args, methodproxy methodproxy) throws throwable {
      final object o = super.invoke(enhanced, method, args);
      return o instanceof abstractserialstateholder ? o : methodproxy.invokesuper(o, args);
    }

    @override
    protected abstractserialstateholder newserialstateholder(object userbean, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      return new cglibserialstateholder(userbean, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
    }
  }


}

如上,代理方法首先会检查 aggressive 是否为 true,如果不满足,再去检查 lazyloadtriggermethods 是否包含当前方法名。这里两个条件只要一个为 true,当前实体类中所有需要延迟加载。aggressive 和 lazyloadtriggermethods 两个变量的值取决于下面的配置。

<setting name="aggressivelazyloading" value="false"/>
<setting name="lazyloadtriggermethods" value="equals,hashcode"/>

然后代理逻辑会检查使用者是不是调用了实体类的 setter 方法,如果调用了,就将该属性对应的 loadpair 从 loadermap 中移除。为什么要这么做呢?答案是:使用者既然手动调用 setter 方法,说明使用者想自定义某个属性的值。此时,延迟加载逻辑不应该再修改该属性的值,所以这里从 loadermap 中移除属性对于的 loadpair。

最后如果使用者调用的是某个属性的 getter 方法,且该属性配置了延迟加载,此时延迟加载逻辑就会被触发。那接下来,我们来看看延迟加载逻辑是怎样实现的的。

public boolean load(string property) throws sqlexception {
    // 从 loadermap 中移除 property 所对应的 loadpair
    loadpair pair = loadermap.remove(property.touppercase(locale.english));
    if (pair != null) {
        // 加载结果
        pair.load();
        return true;
    }
    return false;
}

public void load(final object userobject) throws sqlexception {
    /*
     * 调用 resultloader 的 loadresult 方法加载结果,
     * 并通过 metaresultobject 设置结果到实体类对象中
     */
    this.metaresultobject.setvalue(property, this.resultloader.loadresult());
}

public object loadresult() throws sqlexception {
    // 执行关联查询
    list<object> list = selectlist();
    // 抽取结果
    resultobject = resultextractor.extractobjectfromlist(list, targettype);
    return resultobject;
}

private <e> list<e> selectlist() throws sqlexception {
    executor localexecutor = executor;
    if (thread.currentthread().getid() != this.creatorthreadid || localexecutor.isclosed()) {
        localexecutor = newexecutor();
    }
    try {
        // 通过 executor 就行查询,这个之前已经分析过了
        // 这里的parameterobject和boundsql就是我们之前存放在loadpair中的,现在直接拿来执行了
        return localexecutor.<e>query(mappedstatement, parameterobject, rowbounds.default,
                                      executor.no_result_handler, cachekey, boundsql);
    } finally {
        if (localexecutor != executor) {
            localexecutor.close(false);
        }
    }
}

好了,延迟加载我们基本已经讲清楚了,我们介绍一下另外的一种代理方式

javassistproxyfactory

javassistproxyfactory使用的是javassist方式,直接修改class文件的字节码格式。

import java.lang.reflect.method;
import java.util.list;
import java.util.map;
import java.util.properties;
import java.util.set;

import javassist.util.proxy.methodhandler;
import javassist.util.proxy.proxy;
import javassist.util.proxy.proxyfactory;

import org.apache.ibatis.executor.executorexception;
import org.apache.ibatis.executor.loader.abstractenhanceddeserializationproxy;
import org.apache.ibatis.executor.loader.abstractserialstateholder;
import org.apache.ibatis.executor.loader.resultloadermap;
import org.apache.ibatis.executor.loader.writereplaceinterface;
import org.apache.ibatis.io.resources;
import org.apache.ibatis.logging.log;
import org.apache.ibatis.logging.logfactory;
import org.apache.ibatis.reflection.exceptionutil;
import org.apache.ibatis.reflection.factory.objectfactory;
import org.apache.ibatis.reflection.property.propertycopier;
import org.apache.ibatis.reflection.property.propertynamer;
import org.apache.ibatis.session.configuration;

/**javassistproxy字节码生成代理
 * 1.创建一个代理对象然后将目标对象的值赋值给代理对象,这个代理对象是可以实现其他的接口
 * 2. javassistproxyfactory实现proxyfactory接口createproxy(创建代理对象的方法)
 * @author eduardo macarron
 */
public class javassistproxyfactory implements org.apache.ibatis.executor.loader.proxyfactory {

  /**
   * finalize方法(垃圾回收)
   */
  private static final string finalize_method = "finalize";

  /**
   * writereplace(序列化写出方法)
   */
  private static final string write_replace_method = "writereplace";

  /**
   * 加载proxyfactory, 也就是javassistproxy的入口
   */
  public javassistproxyfactory() {
    try {
      resources.classforname("javassist.util.proxy.proxyfactory");
    } catch (throwable e) {
      throw new illegalstateexception("cannot enable lazy loading because javassist is not available. add javassist to your classpath.", e);
    }
  }

  /**
   * 创建代理
   * @param target 目标对象
   * @param lazyloader 延迟加载map集合(那些属性是需要延迟加载的)
   * @param configuration 配置类
   * @param objectfactory 对象工厂
   * @param constructorargtypes 构造函数类型[]
   * @param constructorargs  构造函数的值[]
   * @return
   */
  @override
  public object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
    return enhancedresultobjectproxyimpl.createproxy(target, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
  }

  public object createdeserializationproxy(object target, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
    return enhanceddeserializationproxyimpl.createproxy(target, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
  }

  @override
  public void setproperties(properties properties) {
      // not implemented
  }

  /**
   * 获取代理对象, 也就是说在执行方法之前首先调用methodhanlder的invoke方法
   * @param type 目标类型
   * @param callback 回调对象
   * @param constructorargtypes 构造函数类型数组
   * @param constructorargs 构造函数值的数组
   * @return
   */
  static object crateproxy(class<?> type, methodhandler callback, list<class<?>> constructorargtypes, list<object> constructorargs) {
    // 创建一个代理工厂类
    // 配置超类
    proxyfactory enhancer = new proxyfactory();
    enhancer.setsuperclass(type);
    //判断是否有writereplace方法,如果没有将这个代理对象实现writereplaceinterface接口,这个接口只有一个writereplace方法
    try {
      type.getdeclaredmethod(write_replace_method);
      // objectoutputstream will call writereplace of objects returned by writereplace
      if (logholder.log.isdebugenabled()) {
        logholder.log.debug(write_replace_method + " method was found on bean " + type + ", make sure it returns this");
      }
    } catch (nosuchmethodexception e) {
      enhancer.setinterfaces(new class[]{writereplaceinterface.class});
    } catch (securityexception e) {
      // nothing to do here
    }

    object enhanced;
    class<?>[] typesarray = constructorargtypes.toarray(new class[constructorargtypes.size()]);
    object[] valuesarray = constructorargs.toarray(new object[constructorargs.size()]);
    try {
      // 根据构造函数创建一个代理对象
      enhanced = enhancer.create(typesarray, valuesarray);
    } catch (exception e) {
      throw new executorexception("error creating lazy proxy.  cause: " + e, e);
    }
    // 设置回调对象
    ((proxy) enhanced).sethandler(callback);
    return enhanced;
  }

  /**
   * 实现javassist的methodhandler接口, 相对于cglib的methodinterceptor
   * 他们接口的方法名也是不一样的,javassist的是invoke, 而cglib是intercept,叫法不同,实现功能是一样的
   */
  private static class enhancedresultobjectproxyimpl implements methodhandler {

    /**
     * 目标类型
     */
    private final class<?> type;
    /**
     * 延迟加载map集合
     */
    private final resultloadermap lazyloader;

    /**
     * 是否配置延迟加载
     */
    private final boolean aggressive;

    /**
     * 延迟加载触发的方法
     */
    private final set<string> lazyloadtriggermethods;

    /**
     * 对象工厂
     */
    private final objectfactory objectfactory;

    /**
     * 构造函数类型数组
     */
    private final list<class<?>> constructorargtypes;

    /**
     * 构造函数类型的值数组
     */
    private final list<object> constructorargs;

    /**
     * 构造函数私有化了
     * @param type
     * @param lazyloader
     * @param configuration
     * @param objectfactory
     * @param constructorargtypes
     * @param constructorargs
     */
    private enhancedresultobjectproxyimpl(class<?> type, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
      this.type = type;
      this.lazyloader = lazyloader;
      this.aggressive = configuration.isaggressivelazyloading();
      this.lazyloadtriggermethods = configuration.getlazyloadtriggermethods();
      this.objectfactory = objectfactory;
      this.constructorargtypes = constructorargtypes;
      this.constructorargs = constructorargs;
    }

    public static object createproxy(object target, resultloadermap lazyloader, configuration configuration, objectfactory objectfactory, list<class<?>> constructorargtypes, list<object> constructorargs) {
      // 获取目标类型
      // 创建一个enhancedresultobjectproxyimpl对象,回调对象
      final class<?> type = target.getclass();
      enhancedresultobjectproxyimpl callback = new enhancedresultobjectproxyimpl(type, lazyloader, configuration, objectfactory, constructorargtypes, constructorargs);
      object enhanced = crateproxy(type, callback, constructorargtypes, constructorargs);
      propertycopier.copybeanproperties(type, target, enhanced);
      return enhanced;
    }

    /**
     * 回调方法
     * @param enhanced 代理对象
     * @param method 方法
     * @param methodproxy 代理方法
     * @param args 入参
     * @return
     * @throws throwable
     */
    @override
    public object invoke(object enhanced, method method, method methodproxy, object[] args) throws throwable {
      //获取方法名称
      final string methodname = method.getname();
      try {
        synchronized (lazyloader) {
          if (write_replace_method.equals(methodname)) {
            //如果方法是writereplace
            object original;
            if (constructorargtypes.isempty()) {
              original = objectfactory.create(type);
            } else {
              original = objectfactory.create(type, constructorargtypes, constructorargs);
            }
            propertycopier.copybeanproperties(type, enhanced, original);
            if (lazyloader.size() > 0) {
              return new javassistserialstateholder(original, lazyloader.getproperties(), objectfactory, constructorargtypes, constructorargs);
            } else {
              return original;
            }
          } else {
            //不是writereplace方法
            // 延迟加载长度大于0, 且不是finalize方法
            // configuration配置延迟加载参数,延迟加载触发的方法包含这个方法
            // 延迟加载所有数据
            if (lazyloader.size() > 0 && !finalize_method.equals(methodname)) {
              if (aggressive || lazyloadtriggermethods.contains(methodname)) {
                lazyloader.loadall();
              } else if (propertynamer.issetter(methodname)) {
                final string property = propertynamer.methodtoproperty(methodname);
                lazyloader.remove(property);
              } else if (propertynamer.isgetter(methodname)) {
                final string property = propertynamer.methodtoproperty(methodname);
                if (lazyloader.hasloader(property)) {
                  lazyloader.load(property);
                }
              }
            }
          }
        }
        return methodproxy.invoke(enhanced, args);
      } catch (throwable t) {
        throw exceptionutil.unwrapthrowable(t);
      }
    }
  }

  private static class enhanceddeserializationproxyimpl extends abstractenhanceddeserializationproxy implements methodhandler {

    private enhanceddeserializationproxyimpl(class<?> type, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      super(type, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
    }

    public static object createproxy(object target, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      final class<?> type = target.getclass();
      enhanceddeserializationproxyimpl callback = new enhanceddeserializationproxyimpl(type, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
      object enhanced = crateproxy(type, callback, constructorargtypes, constructorargs);
      propertycopier.copybeanproperties(type, target, enhanced);
      return enhanced;
    }

    @override
    public object invoke(object enhanced, method method, method methodproxy, object[] args) throws throwable {
      final object o = super.invoke(enhanced, method, args);
      return o instanceof abstractserialstateholder ? o : methodproxy.invoke(o, args);
    }

    @override
    protected abstractserialstateholder newserialstateholder(object userbean, map<string, resultloadermap.loadpair> unloadedproperties, objectfactory objectfactory,
            list<class<?>> constructorargtypes, list<object> constructorargs) {
      return new javassistserialstateholder(userbean, unloadedproperties, objectfactory, constructorargtypes, constructorargs);
    }
  }

  private static class logholder {
    private static final log log = logfactory.getlog(javassistproxyfactory.class);
  }

}

注释已经很清楚了,我就不累述了