jdk8动态代理源码分析

JDK动态代理的实现原理

1)通过实现InvocationHandler接口来自定义自己的InvocationHandler;
2)通过Proxy.getProxyClass获得动态代理类;
3)通过反射机制获得代理类的构造方法,方法签名getConstructor(InvocationHandler.class);
4)通过构造函数获得代理对象并将自定义的InvocationHandler实例对象传为参数传入;
5)通过代理对象调用目标方法;

IHello接口

package com.jpeony.spring.proxy.jdk;
 
public interface IHello {
    void sayHello();
}

HelloImpl接口实现

package com.jpeony.spring.proxy.jdk;
 
public class HelloImpl implements IHello {
    @Override
    public void sayHello() {
        System.out.println("Hello world!");
    }
}

MyInvocationHandler(实现InvocationHandler接口)

package com.jpeony.spring.proxy.jdk;
 
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
 
public class MyInvocationHandler implements InvocationHandler {
 
    /** 目标对象 */
    private Object target;
 
    public MyInvocationHandler(Object target){
        this.target = target;
    }
 
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("------插入前置通知代码-------------");
        // 执行相应的目标方法
        Object rs = method.invoke(target,args);
        System.out.println("------插入后置处理代码-------------");
        return rs;
    }
}

MyProxyTest(Client)

package com.jpeony.spring.proxy.jdk;
 
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;
 
/**
 * 使用JDK动态代理的五大步骤:
 * 1.通过实现InvocationHandler接口来自定义自己的InvocationHandler;
 * 2.通过Proxy.getProxyClass获得动态代理类
 * 3.通过反射机制获得代理类的构造方法,方法签名为getConstructor(InvocationHandler.class)
 * 4.通过构造函数获得代理对象并将自定义的InvocationHandler实例对象传为参数传入
 * 5.通过代理对象调用目标方法
 */
public class MyProxyTest {
    public static void main(String[] args)
            throws NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException {
        // =========================第一种==========================
        // 1、生成$Proxy0的class文件
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        // 2、获取动态代理类
        Class proxyClazz = Proxy.getProxyClass(IHello.class.getClassLoader(),IHello.class);
        // 3、获得代理类的构造函数,并传入参数类型InvocationHandler.class
        Constructor constructor = proxyClazz.getConstructor(InvocationHandler.class);
        // 4、通过构造函数来创建动态代理对象,将自定义的InvocationHandler实例传入
        IHello iHello1 = (IHello) constructor.newInstance(new MyInvocationHandler(new HelloImpl()));
        // 5、通过代理对象调用目标方法
        iHello1.sayHello();
 
        // ==========================第二种=============================
        /**
         * Proxy类中还有个将2~4步骤封装好的简便方法来创建动态代理对象,
         *其方法签名为:newProxyInstance(ClassLoader loader,Class[] instance, InvocationHandler h)
         */
        IHello  iHello2 = (IHello) Proxy.newProxyInstance(IHello.class.getClassLoader(), // 加载接口的类加载器
                new Class[]{IHello.class}, // 一组接口
                new MyInvocationHandler(new HelloImpl())); // 自定义的InvocationHandler
        iHello2.sayHello();
    }
}
image.png

源码分析

以Proxy.newProxyInstance()方法为切入点来剖析代理类的生成及代理方法的调用。

@CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
    // 如果h为空直接抛出空指针异常,之后所有的单纯的判断null并抛异常,都是此方法
        Objects.requireNonNull(h);
    // 拷贝类实现的所有接口
        final Class[] intfs = interfaces.clone();
    // 获取当前系统安全接口
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
        // Reflection.getCallerClass返回调用该方法的方法的调用类;loader:接口的类加载器
        // 进行包访问权限、类加载器权限等检查
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
 
        /*
         * Look up or generate the designated proxy class.
     * 译: 查找或生成指定的代理类
         */
        Class cl = getProxyClass0(loader, intfs);
 
        /*
         * Invoke its constructor with the designated invocation handler.
     * 译: 用指定的调用处理程序调用它的构造函数。
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
       /*
        * 获取代理类的构造函数对象。
        * constructorParams是类常量,作为代理类构造函数的参数类型,常量定义如下:
        * private static final Class[] constructorParams = { InvocationHandler.class };
        */
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
        // 根据代理类的构造函数对象来创建需要返回的代理类对象
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

newProxyInstance()方法帮我们执行了生成代理类----获取构造器----生成代理对象这三步;

生成代理类: Class cl = getProxyClass0(loader, intfs);

获取构造器: final Constructor cons = cl.getConstructor(constructorParams);

生成代理对象: cons.newInstance(new Object[]{h});

Proxy.getProxyClass0()如何生成代理类?

private static Class getProxyClass0(ClassLoader loader,
                                           Class... interfaces) {
    // 接口数不得超过65535个,这么大,足够使用的了
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
 
        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
        // otherwise, it will create the proxy class via the ProxyClassFactory
    // 译: 如果缓存中有代理类了直接返回,否则将由代理类工厂ProxyClassFactory创建代理类
        return proxyClassCache.get(loader, interfaces);
    }

如果缓存中没有代理类,Proxy中的ProxyClassFactory如何创建代理类?从get()方法追踪进去看看。

public V get(K key, P parameter) {// key:类加载器;parameter:接口数组
        // 检查指定类型的对象引用不为空null。当参数为null时,抛出空指针异常。
        Objects.requireNonNull(parameter);
        // 清除已经被GC回收的弱引用
        expungeStaleEntries();
        // 将ClassLoader包装成CacheKey, 作为一级缓存的key
        Object cacheKey = CacheKey.valueOf(key, refQueue);
 
        // lazily install the 2nd level valuesMap for the particular cacheKey
        // 获取得到二级缓存
        ConcurrentMap> valuesMap = map.get(cacheKey);
        // 没有获取到对应的值
        if (valuesMap == null) {
            ConcurrentMap> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }
 
        // create subKey and retrieve the possible Supplier stored by that
        // subKey from valuesMap
        // 根据代理类实现的接口数组来生成二级缓存key
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        // 通过subKey获取二级缓存值
        Supplier supplier = valuesMap.get(subKey);
        Factory factory = null;
        // 这个循环提供了轮询机制, 如果条件为假就继续重试直到条件为真为止
        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue instance
                // 在这里supplier可能是一个Factory也可能会是一个CacheValue
                // 在这里不作判断, 而是在Supplier实现类的get方法里面进行验证
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)
 
            // lazily construct a Factory
            if (factory == null) {
                // 新建一个Factory实例作为subKey对应的值
                factory = new Factory(key, parameter, subKey, valuesMap);
            }
 
            if (supplier == null) {
                // 到这里表明subKey没有对应的值, 就将factory作为subKey的值放入
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    // 到这里表明成功将factory放入缓存
                    supplier = factory;
                }
                // 否则, 可能期间有其他线程修改了值, 那么就不再继续给subKey赋值, 而是取出来直接用
                // else retry with winning supplier
            } else {
                // 期间可能其他线程修改了值, 那么就将原先的值替换
                if (valuesMap.replace(subKey, supplier, factory)) {
                    // successfully replaced
                    // cleared CacheEntry / unsuccessful Factory
                    // with our Factory
                    // 成功将factory替换成新的值
                    supplier = factory;
                } else {
                    // retry with current supplier
                    // 替换失败, 继续使用原先的值
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

get方法中Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));

subKeyFactory调用apply,具体实现在ProxyClassFactory中完成。

ProxyClassFactory.apply()实现代理类创建。

private static final class ProxyClassFactory
        implements BiFunction[], Class>
    {
        // prefix for all proxy class names
    // 统一代理类的前缀名都以$Proxy
        private static final String proxyClassNamePrefix = "$Proxy";
 
        // next number to use for generation of unique proxy class names
    // 使用唯一的编号给作为代理类名的一部分,如$Proxy0,$Proxy1等
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
 
        @Override
        public Class apply(ClassLoader loader, Class[] interfaces) {
 
            Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
         * 验证指定的类加载器(loader)加载接口所得到的Class对象(interfaceClass)是否与intf对象相同
                 */
                Class interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }
                /*
                 * Verify that the Class object actually represents an
                 * interface.
         * 验证该Class对象是不是接口
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /*
                 * Verify that this interface is not a duplicate.
         * 验证该接口是否重复
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }
        // 声明代理类所在包
            String proxyPkg = null;     // package to define proxy class in
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
 
            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
         * 验证所有非公共的接口在同一个包内;公共的就无需处理
             */
            for (Class intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    // 截取完整包名
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) {
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) {
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }
 
            if (proxyPkg == null) {
                // if no non-public proxy interfaces, use com.sun.proxy package
        /*如果都是public接口,那么生成的代理类就在com.sun.proxy包下如果报java.io.FileNotFoundException: com\sun\proxy\$Proxy0.class 
        (系统找不到指定的路径。)的错误,就先在你项目中创建com.sun.proxy路径*/
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }
 
            /*
             * Choose a name for the proxy class to generate.
         * nextUniqueNumber 是一个原子类,确保多线程安全,防止类名重复,类似于:$Proxy0,$Proxy1......
             */
            long num = nextUniqueNumber.getAndIncrement();
        // 代理类的完全限定名,如com.sun.proxy.$Proxy0.calss
            String proxyName = proxyPkg + proxyClassNamePrefix + num;
 
            /*
             * Generate the specified proxy class.
         * 生成类字节码的方法(重点)
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                /*
                 * A ClassFormatError here means that (barring bugs in the
                 * proxy class generation code) there was some other
                 * invalid aspect of the arguments supplied to the proxy
                 * class creation (such as virtual machine limitations
                 * exceeded).
                 */
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

代理类创建真正在ProxyGenerator.generateProxyClass()方法中,方法签名如下:

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);

public static byte[] generateProxyClass(final String name, Class[] interfaces, int accessFlags) {
        ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
        // 真正生成字节码的方法
        final byte[] classFile = gen.generateClassFile();
        // 如果saveGeneratedFiles为true 则生成字节码文件,所以在开始我们要设置这个参数
        // 当然,也可以通过返回的bytes自己输出
        if (saveGeneratedFiles) {
            java.security.AccessController.doPrivileged( new java.security.PrivilegedAction() {
                        public Void run() {
                            try {
                                int i = name.lastIndexOf('.');
                                Path path;
                                if (i > 0) {
                                    Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
                                    Files.createDirectories(dir);
                                    path = dir.resolve(name.substring(i+1, name.length()) + ".class");
                                } else {
                                    path = Paths.get(name + ".class");
                                }
                                Files.write(path, classFile);
                                return null;
                            } catch (IOException e) {
                                throw new InternalError( "I/O exception saving generated file: " + e);
                            }
                        }
                    });
        }
        return classFile;
    }

代理类生成的最终方法是ProxyGenerator.generateClassFile()

private byte[] generateClassFile() {
        /* ============================================================
         * Step 1: Assemble ProxyMethod objects for all methods to generate proxy dispatching code for.
         * 步骤1:为所有方法生成代理调度代码,将代理方法对象集合起来。
         */
        //增加 hashcode、equals、toString方法
        addProxyMethod(hashCodeMethod, Object.class);
        addProxyMethod(equalsMethod, Object.class);
        addProxyMethod(toStringMethod, Object.class);
        // 获得所有接口中的所有方法,并将方法添加到代理方法中
        for (Class intf : interfaces) {
            for (Method m : intf.getMethods()) {
                addProxyMethod(m, intf);
            }
        }
 
        /*
         * 验证方法签名相同的一组方法,返回值类型是否相同;意思就是重写方法要方法签名和返回值一样
         */
        for (List sigmethods : proxyMethods.values()) {
            checkReturnTypes(sigmethods);
        }
 
        /* ============================================================
         * Step 2: Assemble FieldInfo and MethodInfo structs for all of fields and methods in the class we are generating.
         * 为类中的方法生成字段信息和方法信息
         */
        try {
            // 生成代理类的构造函数
            methods.add(generateConstructor());
            for (List sigmethods : proxyMethods.values()) {
                for (ProxyMethod pm : sigmethods) {
                    // add static field for method's Method object
                    fields.add(new FieldInfo(pm.methodFieldName,
                            "Ljava/lang/reflect/Method;",
                            ACC_PRIVATE | ACC_STATIC));
                    // generate code for proxy method and add it
                    // 生成代理类的代理方法
                    methods.add(pm.generateMethod());
                }
            }
            // 为代理类生成静态代码块,对一些字段进行初始化
            methods.add(generateStaticInitializer());
        } catch (IOException e) {
            throw new InternalError("unexpected I/O Exception", e);
        }
 
        if (methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        }
        if (fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        }
 
        /* ============================================================
         * Step 3: Write the final class file.
         * 步骤3:编写最终类文件
         */
        /*
         * Make sure that constant pool indexes are reserved for the following items before starting to write the final class file.
         * 在开始编写最终类文件之前,确保为下面的项目保留常量池索引。
         */
        cp.getClass(dotToSlash(className));
        cp.getClass(superclassName);
        for (Class intf: interfaces) {
            cp.getClass(dotToSlash(intf.getName()));
        }
 
        /*
         * Disallow new constant pool additions beyond this point, since we are about to write the final constant pool table.
         * 设置只读,在这之前不允许在常量池中增加信息,因为要写常量池表
         */
        cp.setReadOnly();
 
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        DataOutputStream dout = new DataOutputStream(bout);
 
        try {
            // u4 magic;
            dout.writeInt(0xCAFEBABE);
            // u2 次要版本;
            dout.writeShort(CLASSFILE_MINOR_VERSION);
            // u2 主版本
            dout.writeShort(CLASSFILE_MAJOR_VERSION);
 
            cp.write(dout);             // (write constant pool)
 
            // u2 访问标识;
            dout.writeShort(accessFlags);
            // u2 本类名;
            dout.writeShort(cp.getClass(dotToSlash(className)));
            // u2 父类名;
            dout.writeShort(cp.getClass(superclassName));
            // u2 接口;
            dout.writeShort(interfaces.length);
            // u2 interfaces[interfaces_count];
            for (Class intf : interfaces) {
                dout.writeShort(cp.getClass(
                        dotToSlash(intf.getName())));
            }
            // u2 字段;
            dout.writeShort(fields.size());
            // field_info fields[fields_count];
            for (FieldInfo f : fields) {
                f.write(dout);
            }
            // u2 方法;
            dout.writeShort(methods.size());
            // method_info methods[methods_count];
            for (MethodInfo m : methods) {
                m.write(dout);
            }
            // u2 类文件属性:对于代理类来说没有类文件属性;
            dout.writeShort(0); // (no ClassFile attributes for proxy classes)
 
        } catch (IOException e) {
            throw new InternalError("unexpected I/O Exception", e);
        }
 
        return bout.toByteArray();
    }

通过addProxyMethod()添加hashcode、equals、toString方法。

private void addProxyMethod(Method var1, Class var2) {
        String var3 = var1.getName();  //方法名
        Class[] var4 = var1.getParameterTypes();   //方法参数类型数组
        Class var5 = var1.getReturnType();    //返回值类型
        Class[] var6 = var1.getExceptionTypes();   //异常类型
        String var7 = var3 + getParameterDescriptors(var4);   //方法签名
        Object var8 = (List)this.proxyMethods.get(var7);   //根据方法签名却获得proxyMethods的Value
        if(var8 != null) {    //处理多个代理接口中重复的方法的情况
            Iterator var9 = ((List)var8).iterator();
            while(var9.hasNext()) {
                ProxyGenerator.ProxyMethod var10 = (ProxyGenerator.ProxyMethod)var9.next();
                if(var5 == var10.returnType) {
                    /*归约异常类型以至于让重写的方法抛出合适的异常类型,我认为这里可能是多个接口中有相同的方法,而这些相同的方法抛出的异常类                      型又不同,所以对这些相同方法抛出的异常进行了归约*/
                    ArrayList var11 = new ArrayList();
                    collectCompatibleTypes(var6, var10.exceptionTypes, var11);
                    collectCompatibleTypes(var10.exceptionTypes, var6, var11);
                    var10.exceptionTypes = new Class[var11.size()];
                    //将ArrayList转换为Class对象数组
                    var10.exceptionTypes = (Class[])var11.toArray(var10.exceptionTypes);
                    return;
                }
            }
        } else {
            var8 = new ArrayList(3);
            this.proxyMethods.put(var7, var8);
        }    
        ((List)var8).add(new ProxyGenerator.ProxyMethod(var3, var4, var5, var6, var2, null));
       /*如果var8为空,就创建一个数组,并以方法签名为key,proxymethod对象数组为value添加到proxyMethods*/
    }

生成的代理对象$Proxy0.class字节码反编译:

package com.sun.proxy;
 
import com.jpeony.spring.proxy.jdk.IHello;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
 
public final class $Proxy0 extends Proxy
  implements IHello // 继承了Proxy类和实现IHello接口
{
  // 变量,都是private static Method  XXX
  private static Method m1;
  private static Method m3;
  private static Method m2;
  private static Method m0;
 
  // 代理类的构造函数,其参数正是是InvocationHandler实例,Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
  public $Proxy0(InvocationHandler paramInvocationHandler)
    throws 
  {
    super(paramInvocationHandler);
  }
 
  // 以下Object中的三个方法
  public final boolean equals(Object paramObject)
    throws 
  {
    try
    {
      return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
  
  // 接口代理方法
  public final void sayHello()
    throws 
  {
    try
    {
      this.h.invoke(this, m3, null);
      return;
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
 
  public final String toString()
    throws 
  {
    try
    {
      return ((String)this.h.invoke(this, m2, null));
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
 
  public final int hashCode()
    throws 
  {
    try
    {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
 
  // 静态代码块对变量进行一些初始化工作
  static
  {
    try
    {
      // 这里每个方法对象 和类的实际方法绑定
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m3 = Class.forName("com.jpeony.spring.proxy.jdk.IHello").getMethod("sayHello", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      return;
    }
    catch (NoSuchMethodException localNoSuchMethodException)
    {
      throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
    }
    catch (ClassNotFoundException localClassNotFoundException)
    {
      throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
    }
  }
}

当代理对象生成后,最后由InvocationHandler的invoke()方法调用目标方法:
在动态代理中InvocationHandler是核心,每个代理实例都具有一个关联的调用处理程序(InvocationHandler)。
所以对代理方法的调用都是通InvocationHadler的invoke来实现中,而invoke方法根据传入的代理对象,
方法和参数来决定调用代理的哪个方法。

方法签名如下:

invoke(Object Proxy,Method method,Object[] args)

从反编译源码分析调用invoke()过程:

从反编译后的源码看$Proxy0类继承了Proxy类,同时实现了IHello接口,即代理类接口,

所以才能强制将代理对象转换为IHello接口,然后调用$Proxy0中的sayHello()方法。

$Proxy0中sayHello()源码:

public final void sayHello()
    throws 
  {
    try
    {
      this.h.invoke(this, m3, null);
      return;
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

this.h.invoke(this, m3, null); this就是$Proxy0对象; m3就是m3 = Class.forName("com.jpeony.spring.proxy.jdk.IHello").getMethod("sayHello", new Class[0]);即是通过全路径名,反射获取的目标对象中的真实方法加参数。
h就是Proxy类中的变量protected InvocationHandler h;

所以成功的调到了InvocationHandler中的invoke()方法,但是invoke()方法在我们自定义的MyInvocationHandler中实现,MyInvocationHandler中的invoke()方法:

@Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("------插入前置通知代码-------------");
        // 执行相应的目标方法
        Object rs = method.invoke(target,args);
        System.out.println("------插入后置处理代码-------------");
        return rs;
    }

所以,绕了半天,终于调用到了MyInvocationHandler中的invoke()方法,从上面的this.h.invoke(this, m3, null);

可以看出,MyInvocationHandler中invoke第一个参数为$Proxy0(代理对象),第二个参数为目标类的真实方法,第三个参数为目标方法参数,因为sayHello()没有参数,所以是null。

到这里,我们真正的实现了通过代理调用目标对象的完全分析,至于InvocationHandler中的invoke()方法就是最后执行了目标方法。到此完成了代理对象生成,目标方法调用。

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