Spring4.3.x 容器中bean的创建过程(2)——实例化Bean
转自http://blog.csdn.net/chyohn/article/details/54967639
概述
在Spring4.3.x 容器中bean的创建过程(1)——走进初始化bean的主要战场一篇中我们找到了初始化bean的主要场所在AbstractAutowireCapableBeanFactory类中的doCreateBean方法中,在这个方法中首先调用createBeanInstance方法创建bean的实例;然后调用populateBean方法设置bean的属性;接着调用initializeBean方法执行Bean后处理器和InitializingBean对象的afterPropertiesSet方法以及init-method方法;最后调用registerDisposableBeanIfNecessary方法注册bean的销毁方法。它的每一步都比较繁琐,所以我把他们分开来探讨,在这一篇我们只讨论第一步bean的实例化过程中会遇到哪些情况,以及如何处理各个情况。
创建对象有两种方式,其一是直接使用构造器创建对象;其二是使用工厂模式间接创建对象,这中方式又分为静态工厂、工厂方法和抽象工厂3类。在spring中,从纯XML配置的角度出发有四种方式,第一种是使用默认构造器,第二种是使用带参构造器,第三种是使用静态工厂,第四种是使用实例工厂。
<!--默认构造器创建bean-->
<bean id="studentServiceDefault" class="com.chyohn.service.StudentService">
<property name="name" value="默认构造器"/>
</bean>
<!-- 指定构造器创建bean-->
<bean id="studentServiceArgs" class="com.chyohn.service.StudentService">
<constructor-arg name="name" value="带参构造器"/>
</bean>
<!-- 静态工厂创建bean -->
<bean id="studentServiceStaticF" class="com.chyohn.factory.StudentServiceFactory" factory-method="createSudentService" />
<!-- 实例工厂创建bean -->
<bean id="studentServiceFactoryBean" class="com.chyohn.factory.StudentServiceFactory"/>
<bean id="studentServiceSimpleF" factory-bean="studentServiceFactoryBean" factory-method="createSudentService">
<constructor-arg name="name" value="简单工厂"/>
</bean>
Spring中不管是工厂方法还是抽象工厂,只要需要实例化工厂对象的都需要通过第四种配置方式创建bean。下面我们看看Spring如何创建这四种配置方式的bean。
实例化bean
AbstractAutowireCapableBeanFactory类中的doCreateBean方法调用它的createBeanInstance方法实例化bean并返回bean的包装器BeanWrapper对象,源码如下。
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// 获取bean的Class对象
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
if (mbd.getFactoryMethodName() != null) {
// 使用指定的工厂方法创建bean示例
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// 再一次创建已经创建过的bean
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// 获取bean的构造器
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
// 通过带参构造器创建bean对象
return autowireConstructor(beanName, mbd, ctors, args);
}
// 通过无参构造器创建bean对象
return instantiateBean(beanName, mbd);
}
createBeanInstance方法所做的事情非常清晰,它根据bean的定义选择bean采用何种方式创建,这个方法把bean的创建分成3种方式,其一是通过指定的工厂创建bean,这个包括使用实例工厂或静态工厂;其二是通过带参构造器创建bean;其三是通过无参构造器创建bean。下面我们分别探讨这三种方式。
1. 使用工厂创建bean
createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的instantiateUsingFactoryMethod方法通过使用工厂方法创建bean实例的入口,源码如下。
protected BeanWrapper instantiateUsingFactoryMethod(
String beanName, RootBeanDefinition mbd, Object[] explicitArgs) {
return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}
instantiateUsingFactoryMethod方法首先创建一个ConstructorResolver对象后,调用这个对象的instantiateUsingFactoryMethod方法来实例化bean。instantiateUsingFactoryMethod方法的源码如下。
public BeanWrapper instantiateUsingFactoryMethod(
final String beanName, final RootBeanDefinition mbd, final Object[] explicitArgs) {
BeanWrapperImpl bw = new BeanWrapperImpl();
// 向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象
this.beanFactory.initBeanWrapper(bw);
Object factoryBean;
Class<?> factoryClass;
boolean isStatic;
String factoryBeanName = mbd.getFactoryBeanName();
if (factoryBeanName != null) {
if (factoryBeanName.equals(beanName)) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"factory-bean reference points back to the same bean definition");
}
// 获取工厂实例
factoryBean = this.beanFactory.getBean(factoryBeanName);
if (factoryBean == null) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"factory-bean '" + factoryBeanName + "' (or a BeanPostProcessor involved) returned null");
}
if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
throw new IllegalStateException("About-to-be-created singleton instance implicitly appeared " +
"through the creation of the factory bean that its bean definition points to");
}
factoryClass = factoryBean.getClass();
isStatic = false;
} else {
// 静态工厂创建bean,必须要提供工厂的全类名
if (!mbd.hasBeanClass()) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
"bean definition declares neither a bean class nor a factory-bean reference");
}
factoryBean = null;
factoryClass = mbd.getBeanClass();
isStatic = true;
}
Method factoryMethodToUse = null;
ArgumentsHolder argsHolderToUse = null;
Object[] argsToUse = null;
// 获取工厂方法的参数
if (explicitArgs != null) {
argsToUse = explicitArgs;
} else {
Object[] argsToResolve = null;
synchronized (mbd.constructorArgumentLock) {
factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
// Found a cached factory method...
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve);
}
}
// 获取工厂方法,和工厂方法所需要的参数
if (factoryMethodToUse == null || argsToUse == null) {
// 工厂对象可能是一个被代理的对象,那么需要获取正确的工厂类全名称
factoryClass = ClassUtils.getUserClass(factoryClass);
// 获取所有待定的工厂方法
Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
List<Method> candidateSet = new ArrayList<Method>();
for (Method candidate : rawCandidates) {
if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
candidateSet.add(candidate);
}
}
Method[] candidates = candidateSet.toArray(new Method[candidateSet.size()]);
AutowireUtils.sortFactoryMethods(candidates);
ConstructorArgumentValues resolvedValues = null;
boolean autowiring = (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Method> ambiguousFactoryMethods = null;
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
} else {
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
LinkedList<UnsatisfiedDependencyException> causes = null;
for (Method candidate : candidates) {
Class<?>[] paramTypes = candidate.getParameterTypes();
if (paramTypes.length >= minNrOfArgs) {
ArgumentsHolder argsHolder;
if (resolvedValues != null) {
try {
String[] paramNames = null;
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
argsHolder = createArgumentArray(
beanName, mbd, resolvedValues, bw, paramTypes, paramNames, candidate, autowiring);
} catch (UnsatisfiedDependencyException ex) {
if (this.beanFactory.logger.isTraceEnabled()) {
this.beanFactory.logger.trace("Ignoring factory method [" + candidate +
"] of bean '" + beanName + "': " + ex);
}
if (causes == null) {
causes = new LinkedList<UnsatisfiedDependencyException>();
}
causes.add(ex);
continue;
}
} else {
// 如果在配置的时候,参数个数必须要与方法的参数个数相同
if (paramTypes.length != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
if (typeDiffWeight < minTypeDiffWeight) {
// 查找到完全匹配的工厂方法
factoryMethodToUse = candidate;
argsHolderToUse = argsHolder;
argsToUse = argsHolder.arguments;
minTypeDiffWeight = typeDiffWeight;
ambiguousFactoryMethods = null;
} else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&
!mbd.isLenientConstructorResolution() &&
paramTypes.length == factoryMethodToUse.getParameterTypes().length &&
!Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {
// 查找到多个可匹配的方法
if (ambiguousFactoryMethods == null) {
ambiguousFactoryMethods = new LinkedHashSet<Method>();
ambiguousFactoryMethods.add(factoryMethodToUse);
}
ambiguousFactoryMethods.add(candidate);
}
}
}
if (factoryMethodToUse == null) {
// 没有查找到可执行的工厂方法,抛出异常
if (causes != null) {
UnsatisfiedDependencyException ex = causes.removeLast();
for (Exception cause : causes) {
this.beanFactory.onSuppressedException(cause);
}
throw ex;
}
List<String> argTypes = new ArrayList<String>(minNrOfArgs);
if (explicitArgs != null) {
for (Object arg : explicitArgs) {
argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
}
} else {
Set<ValueHolder> valueHolders = new LinkedHashSet<ValueHolder>(resolvedValues.getArgumentCount());
valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
valueHolders.addAll(resolvedValues.getGenericArgumentValues());
for (ValueHolder value : valueHolders) {
String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
(value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
argTypes.add(argType);
}
}
String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"No matching factory method found: " +
(mbd.getFactoryBeanName() != null ?
"factory bean '" + mbd.getFactoryBeanName() + "'; " : "") +
"factory method '" + mbd.getFactoryMethodName() + "(" + argDesc + ")'. " +
"Check that a method with the specified name " +
(minNrOfArgs > 0 ? "and arguments " : "") +
"exists and that it is " +
(isStatic ? "static" : "non-static") + ".");
} else if (void.class == factoryMethodToUse.getReturnType()) {
// 查找到的工厂方法的返回类型为void,抛出异常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Invalid factory method '" + mbd.getFactoryMethodName() +
"': needs to have a non-void return type!");
} else if (ambiguousFactoryMethods != null) {
// 查找到多个匹配的工厂方法,抛出异常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Ambiguous factory method matches found in bean '" + beanName + "' " +
"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +
ambiguousFactoryMethods);
}
if (explicitArgs == null && argsHolderToUse != null) {
// 没有传入参数且
argsHolderToUse.storeCache(mbd, factoryMethodToUse);
}
}
try {
// 实例化bean
Object beanInstance;
if (System.getSecurityManager() != null) {
final Object fb = factoryBean;
final Method factoryMethod = factoryMethodToUse;
final Object[] args = argsToUse;
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
// 通过执行工厂方法来创建bean示例
return beanFactory.getInstantiationStrategy().instantiate(
mbd, beanName, beanFactory, fb, factoryMethod, args);
}
}, beanFactory.getAccessControlContext());
} else {
// 通过执行工厂方法来创建bean示例
beanInstance = this.beanFactory.getInstantiationStrategy().instantiate(
mbd, beanName, this.beanFactory, factoryBean, factoryMethodToUse, argsToUse);
}
if (beanInstance == null) {
return null;
}
// 让BeanWraper对象保存bean实例
bw.setBeanInstance(beanInstance);
return bw;
} catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean instantiation via factory method failed", ex);
}
}
instantiateUsingFactoryMethod方法首先获取工厂对象,然后获取工厂方法及其参数,最后调用InstantiationStrategy对象的instantiate方法来创建bean实例。Spring bean工厂默认的InstantiationStrategy对象是CglibSubclassingInstantiationStrategy,这个instantiate方法在CglibSubclassingInstantiationStrategy的父类SimpleInstantiationStrategy中有实现,代码如下。
@Override
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
Object factoryBean, final Method factoryMethod, Object... args) {
try {
// 设置方法的访问权限为可访问
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
ReflectionUtils.makeAccessible(factoryMethod);
return null;
}
});
} else {
ReflectionUtils.makeAccessible(factoryMethod);
}
Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
try {
currentlyInvokedFactoryMethod.set(factoryMethod);
// 执行工厂方法,并返回实例
return factoryMethod.invoke(factoryBean, args);
} finally {
if (priorInvokedFactoryMethod != null) {
currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
} else {
currentlyInvokedFactoryMethod.remove();
}
}
} catch (IllegalArgumentException ex) {
throw new BeanInstantiationException(factoryMethod,
"Illegal arguments to factory method '" + factoryMethod.getName() + "'; " +
"args: " + StringUtils.arrayToCommaDelimitedString(args), ex);
} catch (IllegalAccessException ex) {
throw new BeanInstantiationException(factoryMethod,
"Cannot access factory method '" + factoryMethod.getName() + "'; is it public?", ex);
} catch (InvocationTargetException ex) {
String msg = "Factory method '" + factoryMethod.getName() + "' threw exception";
if (bd.getFactoryBeanName() != null && owner instanceof ConfigurableBeanFactory &&
((ConfigurableBeanFactory) owner).isCurrentlyInCreation(bd.getFactoryBeanName())) {
msg = "Circular reference involving containing bean '" + bd.getFactoryBeanName() + "' - consider " +
"declaring the factory method as static for independence from its containing instance. " + msg;
}
throw new BeanInstantiationException(factoryMethod, msg, ex.getTargetException());
}
}
instantiate方法最主要的目的,就是通过java反射执行工厂方法并返回创建好的实例。
2. 使用带参构造器创建bean
createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的autowireConstructor方法是使用带参构造器创建bean实例的入口,源码如下。
protected BeanWrapper autowireConstructor(
String beanName, RootBeanDefinition mbd, Constructor<?>[] ctors, Object[] explicitArgs) {
return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}
autowireConstructor方法首先创建一个ConstructorResolver对象后,调用这个对象的autowireConstructor方法来实例化bean。autowireConstructor方法的源码如下。
public BeanWrapper autowireConstructor(final String beanName, final RootBeanDefinition mbd,
Constructor<?>[] chosenCtors, final Object[] explicitArgs) {
BeanWrapperImpl bw = new BeanWrapperImpl();
// 向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象
this.beanFactory.initBeanWrapper(bw);
Constructor<?> constructorToUse = null;
ArgumentsHolder argsHolderToUse = null;
Object[] argsToUse = null;
if (explicitArgs != null) {
argsToUse = explicitArgs;
} else {
Object[] argsToResolve = null;
synchronized (mbd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
if (constructorToUse != null && mbd.constructorArgumentsResolved) {
// 从缓存中获取构造器和参数
argsToUse = mbd.resolvedConstructorArguments;
if (argsToUse == null) {
argsToResolve = mbd.preparedConstructorArguments;
}
}
}
if (argsToResolve != null) {
argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve);
}
}
if (constructorToUse == null) {
// 相应的构造器不能从缓存中获取时
// 获取正确的构造器及其参数
boolean autowiring = (chosenCtors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR);
ConstructorArgumentValues resolvedValues = null;
int minNrOfArgs;
if (explicitArgs != null) {
minNrOfArgs = explicitArgs.length;
} else {
ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
resolvedValues = new ConstructorArgumentValues();
minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
}
Constructor<?>[] candidates = chosenCtors;
if (candidates == null) {
Class<?> beanClass = mbd.getBeanClass();
try {
candidates = (mbd.isNonPublicAccessAllowed() ?
beanClass.getDeclaredConstructors() : beanClass.getConstructors());
} catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Resolution of declared constructors on bean Class [" + beanClass.getName() +
"] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
}
}
AutowireUtils.sortConstructors(candidates);
int minTypeDiffWeight = Integer.MAX_VALUE;
Set<Constructor<?>> ambiguousConstructors = null;
LinkedList<UnsatisfiedDependencyException> causes = null;
for (Constructor<?> candidate : candidates) {
Class<?>[] paramTypes = candidate.getParameterTypes();
if (constructorToUse != null && argsToUse.length > paramTypes.length) {
// 找到匹配的构造器,这终止循环
break;
}
if (paramTypes.length < minNrOfArgs) {
// 当前循环的构造器参数个数小于要求的最小参数个数,那么不匹配,继续从匹配下一个
continue;
}
ArgumentsHolder argsHolder;
if (resolvedValues != null) {
// 没有提供构造器参数时执行此段代码
// 这段代码的作用是获取构造器的参数,可能需要自动装配,比如@Autowire、@Value注解的参数
try {
// 从@ConstructorProperties注解中获取构造器的参数名称列表
String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);
if (paramNames == null) {
// 通过ParameterNameDiscoverer对象获取构造器的参数名称列表
ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
if (pnd != null) {
paramNames = pnd.getParameterNames(candidate);
}
}
// 获取构造器参数值列表
// 在这一步会先把未初始化的参数初始化
argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames,
getUserDeclaredConstructor(candidate), autowiring);
} catch (UnsatisfiedDependencyException ex) {
if (this.beanFactory.logger.isTraceEnabled()) {
this.beanFactory.logger.trace(
"Ignoring constructor [" + candidate + "] of bean '" + beanName + "': " + ex);
}
if (causes == null) {
causes = new LinkedList<UnsatisfiedDependencyException>();
}
causes.add(ex);
continue;
}
} else {
// 当前循环的构造器参数个数必须等于提供的参数个数,否则继续查找下一个
if (paramTypes.length != explicitArgs.length) {
continue;
}
argsHolder = new ArgumentsHolder(explicitArgs);
}
int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
if (typeDiffWeight < minTypeDiffWeight) {
// 完全匹配构造器
constructorToUse = candidate;
argsHolderToUse = argsHolder;
argsToUse = argsHolder.arguments;
minTypeDiffWeight = typeDiffWeight;
ambiguousConstructors = null;
} else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
// 多个可匹配的构造器
if (ambiguousConstructors == null) {
ambiguousConstructors = new LinkedHashSet<Constructor<?>>();
ambiguousConstructors.add(constructorToUse);
}
ambiguousConstructors.add(candidate);
}
}
if (constructorToUse == null) {
// 没有找到匹配的构造器
if (causes != null) {
UnsatisfiedDependencyException ex = causes.removeLast();
for (Exception cause : causes) {
this.beanFactory.onSuppressedException(cause);
}
throw ex;
}
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Could not resolve matching constructor " +
"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities)");
} else if (ambiguousConstructors != null && !mbd.isLenientConstructorResolution()) {
// 查找到多个构造器
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Ambiguous constructor matches found in bean '" + beanName + "' " +
"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +
ambiguousConstructors);
}
if (explicitArgs == null) {
// 没有提供构造器参数的情况,保存到缓存中以方便下次构造的时候直接取用
// 其他情况参数个数和类型变动性大、不确认因素多,没有必要存缓存
argsHolderToUse.storeCache(mbd, constructorToUse);
}
}
try {
// 拿到构造器对象和其所需参数后
// 实例化bean
Object beanInstance;
if (System.getSecurityManager() != null) {
final Constructor<?> ctorToUse = constructorToUse;
final Object[] argumentsToUse = argsToUse;
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
// 通过构造器实例化bean
return beanFactory.getInstantiationStrategy().instantiate(
mbd, beanName, beanFactory, ctorToUse, argumentsToUse);
}
}, beanFactory.getAccessControlContext());
} else {
// 通过构造器实例化bean
beanInstance = this.beanFactory.getInstantiationStrategy().instantiate(
mbd, beanName, this.beanFactory, constructorToUse, argsToUse);
}
// 让BeanWraper对象保存bean实例
bw.setBeanInstance(beanInstance);
return bw;
} catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean instantiation via constructor failed", ex);
}
}
autowireConstructor方法首先获取构造器的参数列表对象和构造器对象,最后执行初始化策略器InstantiationStrategy对象的instantiate方法,这个方法的实现在SimpleInstantiationStrategy类中,源码如下。
@Override
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
final Constructor<?> ctor, Object... args) {
if (bd.getMethodOverrides().isEmpty()) {
// 没有配置lookup-method或者replaced-method标签或者@Lookup注解
if (System.getSecurityManager() != null) {
// use own privileged to change accessibility (when security is on)
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
ReflectionUtils.makeAccessible(ctor);
return null;
}
});
}
// 通过BeanUtils直接使用构造器对象实例化bean
return BeanUtils.instantiateClass(ctor, args);
} else {
// 生成CGLIB创建的子类对象
return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
}
}
如果bean的配置没有lookup-method或者replaced-method标签或者@Lookup注解,则直接使用构造器创建bean,否则调用SimpleInstantiationStrategy类或者其子类的instantiateWithMethodInjection方法。这个方法在SimpleInstantiationStrategy类的实现如下。
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
Constructor<?> ctor, Object... args) {
throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");
}
下面是CglibSubclassingInstantiationStrategy类实现的instantiateWithMethodInjection方法的源码。
@Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
Constructor<?> ctor, Object... args) {
// 通过CGLIB生成一个子类对象
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}
CglibSubclassCreator类是CglibSubclassingInstantiationStrategy的私有静态类,下面是它的instantiate方法源码。
public Object instantiate(Constructor<?> ctor, Object... args) {
// 通过Cglib创建一个代理类
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor == null) {
// 没有提供构造器对象,通过BeanUtils工具类使用默认构造器创建一个bean实例
instance = BeanUtils.instantiateClass(subclass);
} else {
try {
// 获取代理类对应的构造器对象,并实例化bean
Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
instance = enhancedSubclassConstructor.newInstance(args);
} catch (Exception ex) {
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
}
}
// 为了避免memory leaks异常,直接在bean实例上设置回调对象
Factory factory = (Factory) instance;
factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
return instance;
}
CglibSubclassCreator类对象通过调用它的createEnhancedSubclass方法来创建一个Cglib代理的子类,源码如下。
private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(beanDefinition.getBeanClass());
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
if (this.owner instanceof ConfigurableBeanFactory) {
ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();
enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));
}
enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
enhancer.setCallbackTypes(CALLBACK_TYPES);
return enhancer.createClass();
}
3. 使用无参构造器创建bean
createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的instantiateBean方法是使用无参构造器创建bean实例的入口,源码如下。
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
} else {
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
// 创建BeanWrapper对象
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
// 初始化BeanWrapper,向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象
initBeanWrapper(bw);
return bw;
} catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
instantiateBean方法通过调用InstantiationStrategy对象的instantiateBean方法创建bean实例,这个instantiateBean方法在SimpleInstantiationStrategy类中的实现代码如下。
@Override
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (bd.getMethodOverrides().isEmpty()) {
// 没有配置lookup-method或者replaced-method标签或者@Lookup注解
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
// 获取默认构造器对象
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
@Override
public Constructor<?> run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
} else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
// 缓存默认构造器对象
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
} catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
return BeanUtils.instantiateClass(constructorToUse);
} else {
// 生成CGLIB创建的子类对象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
关于生成CGLIB创建的子类对象调用调用的方法和使用带参构造器实例化bean一节的一样,这里就不重复说了。
总结
(1)Spring通过实例化策略接口InstantiationStrategy的派生类创建bean,这个接口的源码如下。
public interface InstantiationStrategy {
/**
* 使用默认构造器创建bean实例
* @param bd bean定义信息
* @param beanName bean名称,可以为空
* @param owner bean工厂
* @return
* @throws BeansException
*/
Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner)
throws BeansException;
/**
* 通过指定的构造器创建bean实例
* @param bd bean定义信息
* @param beanName bean名称,可以为空
* @param owner bean工厂
* @param ctor 构造器对象
* @param args 构造器所需参数
* @return
* @throws BeansException
*/
Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
Constructor<?> ctor, Object... args) throws BeansException;
/**
* 根据指定的工厂方法创建bean实例
* @param bd bean的定义信息
* @param beanName bean名称,可以为空
* @param owner bean工厂
* @param factoryBean 工厂对象,如果为静态工厂,此参数为null
* @param factoryMethod 工厂方法对象
* @param args 工厂方法所需参数
* @return
* @throws BeansException
*/
Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
Object factoryBean, Method factoryMethod, Object... args) throws BeansException;
}
Spring默认的实例bean策略类为CglibSubclassingInstantiationStrategy。
(2)使用了lookup-method或者replaced-method标签或者@Lookup注解的bean,Spring使用Cglib的Enhancer类创建它们的代理子类。
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