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JDK动态代理解析

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本文是对jdk动态代理的一些总结。

首先我们生成的时候调用了Proxy.newInstance()这个方法，这个方法会调用getProxyCLass0()方法，而getProxyCLass0()是从ProxyClassFactory中生成proxy代码，而ProxyClassFactory又是使用ProxyGenerator来生成的代码，然后生成的字节码，使用反射来new一个实例。
整个调用大致就是 newInstance() -> getProxyCLass0() -> ProxyClassFactory -> ProxyGenerator -> generateProxyClass

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public class DynamicClient {
    public static void main(String[] args) {
        Subject subject = (Subject) Proxy.newProxyInstance(DynamicClient.class.getClassLoader(),
                new Class[]{Subject.class},
                new JDKProxySubject(new RealSubject()));
        subject.request();
    }
}

我们使用newProxyInstance新建一个实例，进入源码中查看会发现源码如下：

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@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.
     */
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * Invoke its constructor with the designated invocation handler.
     */
    try {
        if (sm != null) {
            checkNewProxyPermission(Reflection.getCallerClass(), cl);
        }

        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;
                }
            });
        }
        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);
    }
}

在源码中有这行代码

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//寻找或生成制定的代理类
Class<?> cl = getProxyClass0(loader, intfs);

进入这个函数，源码如下：

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 /**
 * Generate a proxy class.  Must call the checkProxyAccess method
 * to perform permission checks before calling this.
 */
private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    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
    return proxyClassCache.get(loader, interfaces);
}

发现函数会先从缓存中获取，如果缓存中不存在，则生成一个放入缓存

缓存如下：

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/**
 * a cache of proxy classes
 */
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
    proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

由此可见当不存在的时候是由ProxyClassFactory()来生成的

然后我们看以下这个Factory的源码

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/**
    * A factory function that generates, defines and returns the proxy class given
    * the ClassLoader and array of interfaces.
    */
private static final class ProxyClassFactory
    implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
    // prefix for all proxy class names
    private static final String proxyClassNamePrefix = "$Proxy";

    // next number to use for generation of unique proxy class names
    private static final AtomicLong nextUniqueNumber = new AtomicLong();

    @Override
    public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

        Map<Class<?>, 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.
                */
            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.
                */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException(
                    interfaceClass.getName() + " is not an interface");
            }
            /*
                * Verify that this interface is not a duplicate.
                */
            if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                throw new IllegalArgumentException(
                    "repeated interface: " + interfaceClass.getName());
            }
        }

        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.
            */
        for (Class<?> intf : interfaces) {
            int flags = intf.getModifiers();
            if (!Modifier.isPublic(flags)) {
                accessFlags = Modifier.FINAL;
                String name = intf.getName();
                int n = name.lastIndexOf('.');
                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
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }

        /*
            * Choose a name for the proxy class to generate.
            */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;

        /*
            * Generate the specified proxy class.
            */
        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());
        }
    }
}

在这个类的源码中的apply函数可以看到这几行代码生成字节码

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/*
 * Generate the specified proxy class.
 */
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
    proxyName, interfaces, accessFlags);

下面我们在调用接口的类中添加这个系统配置

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System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

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public class DynamicClient {
    public static void main(String[] args) {
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        Subject subject = (Subject) Proxy.newProxyInstance(DynamicClient.class.getClassLoader(),
                new Class[]{Subject.class},
                new JDKProxySubject(new RealSubject()));
        subject.request();
    }
}

之后在我们运行后会发现项目根路径中出现了下面这个文件

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com/sun/proxy/$Proxy0.class

这个就是生成的字节码文件
进行反编译之后，代码如下

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//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.sun.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import xyz.suiwo.demo.Subject;

public final class $Proxy0 extends Proxy implements Subject {
    private static Method m1;
    private static Method m4;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    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 void hello() throws  {
        try {
            super.h.invoke(this, m4, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    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 void request() throws  {
        try {
            super.h.invoke(this, m3, (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);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m4 = Class.forName("xyz.suiwo.demo.Subject").getMethod("hello");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("xyz.suiwo.demo.Subject").getMethod("request");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

然后我们大致阅读以下反编译的代码，就会有大致了解了，所以我们多添加一个接口的时候，动态代理就会在解析成字节码文件的时候动态生成代理。但是通过静态代理，新添加多少个类就需要手动添加多少次