动态代理理论及Mybatis实战
动态代理在我们平时编程并不常用，但因为Spring AOP以及大量框架均使用动态代理机制。
所以必须要掌握。

动态代理其本质是：PrxoyGenerator.generateProxyClass生成字节码文件，然后通defineClass0加载这个类
说白了是重生成了一个字节码文件。

那先看下源码，在来简单实践一下

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@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException
{
    Objects.requireNonNull(h);

    //克隆一份
    final Class<?>[] intfs = interfaces.clone();
    final SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
    }

    //通过getProxyClass0方法动态生成Class类
    Class<?> cl = getProxyClass0(loader, intfs);

    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);
    }
}

该方法首先创建了一个接口的克隆，之后检查权限。
最重要的一步是Class<?> cl = getProxyClass0(loader, intfs)
Java通过该方法中proxyClassCache.get()方法拿到生成的字节码

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private static Class<?> getProxyClass0(ClassLoader loader,
                                        Class<?>... interfaces) {
     //首先，限制一个类接口不能多余65535
     if (interfaces.length > 65535) {
         throw new IllegalArgumentException("interface limit exceeded");
     }

     // private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
     //  proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
     return proxyClassCache.get(loader, interfaces);
 }

proxyClassCache.get()方法的部分实现

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//最终通过 supplier.get()返回对象
while (true) {
  if (supplier != null) {
    // supplier might be a Factory or a CacheValue<V> instance
    V value = supplier.get();
    if (value != null) {
        return value;
    }
  }

  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);
    }
  }
}
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//supplier.get()的实现
public synchronized V get() { // serialize access
    Supplier<V> supplier = valuesMap.get(subKey);
    if (supplier != this) {
        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);

    // put into reverseMap
    reverseMap.put(cacheValue, Boolean.TRUE);

    // try replacing us with CacheValue (this should always succeed)
    if (!valuesMap.replace(subKey, this, cacheValue)) {
        throw new AssertionError("Should not reach here");
    }

    // successfully replaced us with new CacheValue -> return the value
    // wrapped by it
    return value;
}

之后回过头看apply方法（value = Objects.requireNonNull(valueFactory.apply(key, parameter));）
找到类：ProxyClassFactory

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//·········省略部分验证代码
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) {
        throw new IllegalArgumentException(e.toString());
    }

其关键部分位于PrxoyGenerator.generateProxyClass生成代理类字节码文件。

之后返回newProxyInstance方法，看这个类如何被实例化，

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private static final Class<?>[] constructorParams = { InvocationHandler.class };
····
final Constructor<?> cons = cl.getConstructor(constructorParams);
return cons.newInstance(new Object[]{h});

首先，获取了带参数InvocationHandler类的构造器，然后通过构造器，往里看最后通过调用NativeConstructorAccessorImpl的本地方法实例化了这个类,这样就完成了整个代理过程

接下来看下一个实践，知道原理还是要用一下：

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public interface Subject {
  //先做一个接口，里面只有一个方法
	void sayHello(String str);
}
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import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

//实现InvocationHandler接口
public class Invoke implements InvocationHandler {
	//这个就是我们要代理的真实对象
	private Object subject;

	//构造方法，给我们要代理的真实对象赋初值
	public Invoke(Object subject) {
		this.subject = subject;
	}

	@Override
	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //doSomeThing
		System.out.println("即将被代理的方法");
		method.invoke(subject, args);
		return null;
	}
}

以上就完成了代理类的构建，接下倆我们去调用一下

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import java.lang.reflect.Proxy;

public class doMain {

	public static void main(String[] args){
		Subject subjectImpl = str -> System.out.println("Hello:" + str);

		Invoke invoke = new Invoke(subjectImpl);

		Subject subject = (Subject) Proxy.newProxyInstance(subjectImpl.getClass().getClassLoader(),         subjectImpl.getClass().getInterfaces(), invoke);

		//并未直接调用subjectImpl，而是调用生成的代理类
		subject.sayHello("test");
	}
}
/* 控制台输出：
* 即将被代理的方法
* Hello:test
*/