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动态代理玩不明白?别紧张,你只是缺少这个demo

互联网 diligentman 1周前 (10-15) 12次浏览

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概念

JDK动态代理与静态代理是相对的,静态代理是在编译期就确定了代理类和被代理类的关系,且生成代理类。而且静态代理要求被代理的接口必须有其实现,因为代理类实现接口方法实际上是调用接口实现类的方法实现的。而动态代理是在运行期利用JVM的反射机制生成代理类,这里是直接生成类的字节码,然后通过类加载器载入JAVA虚拟机执行。

demo

不做赘述,直接上demo。

被代理类接口

public interface ISubject {
    void doSomething();
}

被代理类实现

public class SubjectImpl implements ISubject {

    @Override
    public void doSomething() {
        System.out.println("doSomething...");
   }
}

InvocationHandler工具

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
​
public class JDKDynamicProxy implements InvocationHandler{
​
    private Object target;
​
    public JDKDynamicProxy(){
   }
​
    public JDKDynamicProxy(Object target){
        this.target=target;
   }
​
    public <T>T getProxy(){
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        // return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
        return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
   }
​
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before...");
        Object result=method.invoke(target,args);
        System.out.println("after...");
        return result;
   }
}

测试类

public class TestProxy {
​
    public static void main(String[] args) {
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");
​
        ISubject subject=new JDKDynamicProxy(new SubjectImpl()).getProxy();
        //ISubject subject= (ISubject) Proxy.newProxyInstance(subjectImpl.getClass().getClassLoader(),subjectImpl.getClass().getInterfaces(),new JDKDynamicProxy(subjectImpl));
        //ISubject subject= (ISubject) Proxy.newProxyInstance(ISubject.class.getClassLoader(),new Class[]{ISubject.class},new JDKDynamicProxy(subjectImpl));
        subject.doSomething();
   }
}

运行结果

before…
doSomething…
after…

生成的代理类文件

//
// 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;
​
public final class $Proxy0 extends Proxy implements ISubject {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    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 doSomething() throws {
        try {
            super.h.invoke(this, m3, (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 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"));
            m3 = Class.forName("ISubject").getMethod("doSomething");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
       } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
       } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
       }
   }
}

原理

流程

获取代理类——调用Proxy.newProxyInstance获取代理类的Class实例getProxyClass0(loader, intfs)获取代理类实例的构造方法,并确保其访问权限final Constructor cons = cl.getConstructor(constructorParams)利用反射机制调用构造方法,返回代理类实例,参数是调用Proxy.newProxyInstance时传进来的InvocationHandler实例h 。cons.newInstance(new Object[]{h})

方法实现前面利用反射机制调用代理类的构造方法时传入了InvocationHandler实例h ,代理类$Proxy0的构造方法如下:

public $Proxy0(InvocationHandler var1) throws {
    super(var1);
}

这是调用父类的构造函数,并传入参数h。所以在获取到代理类实例并执行方法时如测试类的subject.doSomething(),实际调用的是代理类内的doSomething(),如下所示。

public final void doSomething() throws {
    try {
        super.h.invoke(this, m3, (Object[])null);
   } catch (RuntimeException | Error var2) {
        throw var2;
   } catch (Throwable var3) {
        throw new UndeclaredThrowableException(var3);
   }
}

所以JDK动态代理的接口方法实现逻辑是完全由InvocationHandler实例的invoke方法决定的 。

获取代理类的Class实例探究

getProxyClass0(loader, intfs)方法

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);
}

在Proxy类getProxyClass0(loader, intfs)方法中并无核心代码,主要是方法最后一行去缓存对象中获取代理类实例。

java.lang.reflect.WeakCache 类private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
   proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
proxyClassCache是Proxy类的静态变量,调用WeakCache类的构造函数初始化时,传入keyFactoryproxyClassFactory两个实例。注意WeakCache对象的数据结构包含两级键值。接下来就进入WeakCache类的get方法查看一番。

WeakCache类get方法代码

public V get(K key, P parameter) {
    Objects.requireNonNull(parameter);
    expungeStaleEntries();
    Object cacheKey = CacheKey.valueOf(key, refQueue);
    // lazily install the 2nd level valuesMap for the particular cacheKey
    ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
    if (valuesMap == null) {
        ConcurrentMap<Object, Supplier<V>> oldValuesMap
            = map.putIfAbsent(cacheKey,
                              valuesMap = new ConcurrentHashMap<>());
        if (oldValuesMap != null) {
            valuesMap = oldValuesMap;
       }
   }
​
    // create subKey and retrieve the possible Supplier<V> stored by that
    // subKey from valuesMap
    Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
    Supplier<V> supplier = valuesMap.get(subKey);
    Factory factory = null;
​
    while (true) {
        if (supplier != null) {
            // supplier might be a Factory or a CacheValue<V> instance
            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) {
            factory = new Factory(key, parameter, subKey, valuesMap);
       }
​
        if (supplier == null) {
            supplier = valuesMap.putIfAbsent(subKey, factory);
            if (supplier == null) {
                // successfully installed Factory
                supplier = factory;
           }
            // else retry with winning supplier
       } else {
            if (valuesMap.replace(subKey, supplier, factory)) {
                // successfully replaced
                // cleared CacheEntry / unsuccessful Factory
                // with our Factory
                supplier = factory;
           } else {
                // retry with current supplier
                supplier = valuesMap.get(subKey);
           }
       }
   }
}

​get方法核心代码解释获取一级和二级键。

//根据类加载器计算一级键 cacheKey=f(loader);
Object cacheKey = CacheKey.valueOf(key, refQueue);
​
//根据接口计算二级键 subKey=g(interfaces);
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
​注意这里的subKeyFactory是Proxy初始化WeakCache类对象proxyClassCache时的入参new KeyFactory();KeyFactory是Proxy的内部类。不妨进入apply简单看一下:@Override
public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {
    switch (interfaces.length) {
        case 1: return new Key1(interfaces[0]); // the most frequent
        case 2: return new Key2(interfaces[0], interfaces[1]);
        case 0: return key0;
        default: return new KeyX(interfaces);
   }
}

​循环 接下来是一个while循环,第一次循环前factory和supplier都为null ,所以循环体内只是做了一些初始化的工作,并将factory赋值给了supplier(Factory类是WeakCache的内部类,且继承了Supplier类)。第二次循环时,执行V value = supplier.get(),实际上就是调用factory.get()方法获取动态代理类的Class实例 。factory.get()方法方法完整代码如下:

@Override
public synchronized V get() { // serialize access
    // re-check
    Supplier<V> supplier = valuesMap.get(subKey);
    if (supplier != this) {
        // something changed while we were waiting:
        // might be that we were replaced by a CacheValue
        // or were removed because of failure ->
        // return null to signal WeakCache.get() to retry
        // the loop
        return null;
   }
    // else still us (supplier == this)
​
    // create new value
    V value = null;
    try {
        value = Objects.requireNonNull(valueFactory.apply(key, parameter));
   } finally {
        if (value == null) { // remove us on failure
            valuesMap.remove(subKey, this);
       }
   }
    // the only path to reach here is with non-null value
    assert value != null;
​
    // wrap value with CacheValue (WeakReference)
    CacheValue<V> cacheValue = new CacheValue<>(value);
​
    // try replacing us with CacheValue (this should always succeed)
    if (valuesMap.replace(subKey, this, cacheValue)) {
        // put also in reverseMap
        reverseMap.put(cacheValue, Boolean.TRUE);
   } else {
        throw new AssertionError("Should not reach here");
   }
​
    // successfully replaced us with new CacheValue -> return the value
    // wrapped by it
    return value;
}

​第4行代码Supplier supplier = valuesMap.get(subKey)通过subKey获取方才的supplier也就是factory。然后在第18行调用了valueFactory.apply方法创建动态代理类并将结果赋值给变量value。 valueFactory是初始化WeakCache对象调用构造方法时传入的参数 即Prxoy的内部类ProxyClassFactory对象。

ProxyClassFactory类进入apply方法

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());
   }
}

​该方法前边包括一些校验、计算出访问标志和包名、代理类定名 ,然后在73行创建代理类的字节码文件,并defineClass0将字节码文件加载到虚拟机和返回代理类的Class实例。

如上就是第一次创建代理类的大致流程。

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