动态代理源码解析

接口：

public interface Person {
    String getString();
}

实现类：

public class Student implements Person{
    public String getString(){
        return "sss";
    }
}

 

InvocationHandler实现类：

public class StudentInvocationHandler<T> implements InvocationHandler {
    T target;
    public StudentInvocationHandler(T target) {
        this.target = target;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        return method.invoke(target,args);
    }
}

测试类：

public class Test {
    public static void main(String[] args) {
        Student student=new Student();
        StudentInvocationHandler handler = new StudentInvocationHandler<Person>(student);
        //生成动态代理
        Person person =(Person) Proxy.newProxyInstance(Person.class.getClassLoader(), new Class<?>[]{Person.class}, handler);
        System.out.println(person.getString());
    }
}

 

生成的代理类在debug模式下如下，可以看到person值为代理类$Proxy0,是Student的代理类，此类的特点是和Student一样都实现了Person接口。

 

可以通过以下方式获取$Proxy0的源码：

public static void main(String[] args)throws Exception{
        byte[] proxyClass=ProxyGenerator.generateProxyClass("$Proxy0",Student.class.getInterfaces());
        //将代理类person的源码输出到文件，然后用IDEA等工具打开查看
        File file=new File("E:/$Proxy0.class");
        FileOutputStream fileOutputStream=new FileOutputStream(file);
        fileOutputStream.write(proxyClass);
        fileOutputStream.close();
    }

 代理类的结构如下：

$Proxy0的源码如下，可见代理类实现了Person接口并且继承了Proxy类，当执行代理类的getString方法时，就会执行StudentInvocationHandler的invoke方法

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
import DynamicProxy.Person;
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 Person {
    private static Method m1;
    private static Method m2;
    private static Method m0;
    private static Method m3;
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }
    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }
    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    public final String getString() throws  {
        try {
            return (String)super.h.invoke(this, m3, (Object[])null);//会调用父类（Proxy类）的h(StudentInvocationHandler)的invoike方法
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m3 = Class.forName("DynamicProxy.Person").getMethod("getString");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

源码解析：Proxy.newProxyInstance
此方法为指定接口返回代理类的一个实例

public class Proxy implements java.io.Serializable {
    private static final long serialVersionUID = -2222568056686623797L;
    /** parameter types of a proxy class constructor */
    private static final Class<?>[] constructorParams =
        { InvocationHandler.class };
     /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;   

 /**
     * Returns an instance of a proxy class for the specified interfaces
     * that dispatches method invocations to the specified invocation
     * handler.
     *
     * <p>{@code Proxy.newProxyInstance} throws
     * {@code IllegalArgumentException} for the same reasons that
     * {@code Proxy.getProxyClass} does.
     *
     * @param   loader the class loader to define the proxy class
     * @param   interfaces the list of interfaces for the proxy class
     *          to implement
     * @param   h the invocation handler to dispatch method invocations to
     * @return  a proxy instance with the specified invocation handler of a
     *          proxy class that is defined by the specified class loader
     *          and that implements the specified interfaces
     * @throws  IllegalArgumentException if any of the restrictions on the
     *          parameters that may be passed to {@code getProxyClass}
     *          are violated
     * @throws  SecurityException if a security manager, <em>s</em>, is present
     *          and any of the following conditions is met:
     *          <ul>
     *          <li> the given {@code loader} is {@code null} and
     *               the caller's class loader is not {@code null} and the
     *               invocation of {@link SecurityManager#checkPermission
     *               s.checkPermission} with
     *               {@code RuntimePermission("getClassLoader")} permission
     *               denies access;</li>
     *          <li> for each proxy interface, {@code intf},
     *               the caller's class loader is not the same as or an
     *               ancestor of the class loader for {@code intf} and
     *               invocation of {@link SecurityManager#checkPackageAccess
     *               s.checkPackageAccess()} denies access to {@code intf};</li>
     *          <li> any of the given proxy interfaces is non-public and the
     *               caller class is not in the same {@linkplain Package runtime package}
     *               as the non-public interface and the invocation of
     *               {@link SecurityManager#checkPermission s.checkPermission} with
     *               {@code ReflectPermission("newProxyInPackage.{package name}")}
     *               permission denies access.</li>
     *          </ul>
     * @throws  NullPointerException if the {@code interfaces} array
     *          argument or any of its elements are {@code null}, or
     *          if the invocation handler, {@code h}, is
     *          {@code null}
     */
    @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);
        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.（查找或者生产代理类的class）
         */
        Class<?> cl = getProxyClass0(loader, intfs);
        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
         //根据构造器参数类型获取Constructor<?>类,此测试中constructorParams的值为InvocationHandler.class
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //根据构造器和构造器参数生成代理类实例，此测试中h的值为
            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);
        }
    }
}