json解析的原理

众所周知，json格式的字符串，有很多公用包可以做到，那具体怎么做的呢？

其实猜也可以猜到，大概是通过类Class的反射，获取每个属性，然后输出成响应格式的文本，来看看fastjson-1.2.4具体实现：

    public static final String toJSONString(Object object) {
        return toJSONString(object, new SerializerFeature[0]);
    }

    public static final String toJSONString(Object object, SerializerFeature... features) {
        SerializeWriter out = new SerializeWriter();

        try {
            JSONSerializer serializer = new JSONSerializer(out);
            for (com.alibaba.fastjson.serializer.SerializerFeature feature : features) {
                serializer.config(feature, true);
            }

            serializer.write(object);

            return out.toString();
        } finally {
            out.close();
        }
    }

 

    public final void write(Object object) {
        if (object == null) {
            out.writeNull();
            return;
        }

        Class<?> clazz = object.getClass();
        ObjectSerializer writer = getObjectWriter(clazz);

        try {
            writer.write(this, object, null, null, 0);
        } catch (IOException e) {
            throw new JSONException(e.getMessage(), e);
        }
    }

 

	public ObjectSerializer getObjectWriter(Class<?> clazz) {
        ObjectSerializer writer = get(clazz);

        if (writer == null) {
            try {
                final ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
                for (Object o : ServiceLoader.load(AutowiredObjectSerializer.class, classLoader)) {
                    if (!(o instanceof AutowiredObjectSerializer)) {
                        continue;
                    }

                    AutowiredObjectSerializer autowired = (AutowiredObjectSerializer) o;
                    for (Type forType : autowired.getAutowiredFor()) {
                        put(forType, autowired);
                    }
                }
            } catch (ClassCastException ex) {
                // skip
            }

            writer = get(clazz);
        }

        if (writer == null) {
            final ClassLoader classLoader = JSON.class.getClassLoader();
            if (classLoader != Thread.currentThread().getContextClassLoader()) {
                try {
                    for (Object o : ServiceLoader.load(AutowiredObjectSerializer.class, classLoader)) {

                        if (!(o instanceof AutowiredObjectSerializer)) {
                            continue;
                        }

                        AutowiredObjectSerializer autowired = (AutowiredObjectSerializer) o;
                        for (Type forType : autowired.getAutowiredFor()) {
                            put(forType, autowired);
                        }
                    }
                } catch (ClassCastException ex) {
                    // skip
                }

                writer = get(clazz);
            }
        }

        if (writer == null) {
            if (Map.class.isAssignableFrom(clazz)) {
                put(clazz, MapSerializer.instance);
            } else if (List.class.isAssignableFrom(clazz)) {
                put(clazz, ListSerializer.instance);
            } else if (Collection.class.isAssignableFrom(clazz)) {
                put(clazz, CollectionSerializer.instance);
            } else if (Date.class.isAssignableFrom(clazz)) {
                put(clazz, DateSerializer.instance);
            } else if (JSONAware.class.isAssignableFrom(clazz)) {
                put(clazz, JSONAwareSerializer.instance);
            } else if (JSONSerializable.class.isAssignableFrom(clazz)) {
                put(clazz, JSONSerializableSerializer.instance);
            } else if (JSONStreamAware.class.isAssignableFrom(clazz)) {
                put(clazz, JSONStreamAwareSerializer.instance);
            } else if (clazz.isEnum() || (clazz.getSuperclass() != null && clazz.getSuperclass().isEnum())) {
                put(clazz, EnumSerializer.instance);
            } else if (clazz.isArray()) {
                Class<?> componentType = clazz.getComponentType();
                ObjectSerializer compObjectSerializer = getObjectWriter(componentType);
                put(clazz, new ArraySerializer(componentType, compObjectSerializer));
            } else if (Throwable.class.isAssignableFrom(clazz)) {
                put(clazz, new ExceptionSerializer(clazz));
            } else if (TimeZone.class.isAssignableFrom(clazz)) {
                put(clazz, TimeZoneCodec.instance);
            } else if (Appendable.class.isAssignableFrom(clazz)) {
                put(clazz, AppendableSerializer.instance);
            } else if (Charset.class.isAssignableFrom(clazz)) {
                put(clazz, CharsetCodec.instance);
            } else if (Enumeration.class.isAssignableFrom(clazz)) {
                put(clazz, EnumerationSeriliazer.instance);
            } else if (Calendar.class.isAssignableFrom(clazz)) {
                put(clazz, CalendarCodec.instance);
            } else if (Clob.class.isAssignableFrom(clazz)) {
                put(clazz, ClobSeriliazer.instance);
            } else {
                boolean isCglibProxy = false;
                boolean isJavassistProxy = false;
                for (Class<?> item : clazz.getInterfaces()) {
                    if (item.getName().equals("net.sf.cglib.proxy.Factory")
                        || item.getName().equals("org.springframework.cglib.proxy.Factory")) {
                        isCglibProxy = true;
                        break;
                    } else if (item.getName().equals("javassist.util.proxy.ProxyObject")) {
                        isJavassistProxy = true;
                        break;
                    }
                }

                if (isCglibProxy || isJavassistProxy) {
                    Class<?> superClazz = clazz.getSuperclass();

                    ObjectSerializer superWriter = getObjectWriter(superClazz);
                    put(clazz, superWriter);
                    return superWriter;
                }

                if (Proxy.isProxyClass(clazz)) {
                    put(clazz, createJavaBeanSerializer(clazz));
                } else {
                    put(clazz, createJavaBeanSerializer(clazz));
                }
            }

            writer = get(clazz);
        }
        return writer;
    }

 看到这里就差不多明白了，就是根据其不同类型，拿到具体的ObjectSerializer子类，来做不同的输入操作。