上篇文章研究了retrofit的源码，提到了一个重要的概念动态代理，这是一个很重要的知识点，打算详细梳理一下。
相比于静态代理，动态代理的好处是可以让接口里的方法统一做一些处理，而不必要手动添加多个代理类。
先来看一个使用例子，首先建一个接口类，带2个方法：
public interface ServiceApi {
void login();
void loginOut();
}
来一个实现类：
public class ServiceApiImp implements ServiceApi{
@Override
public void login() {
Log.d("TAG","登录了");
}
@Override
public void loginOut() {
Log.d("TAG","退出登录了");
}
}
再来一个InvocationHandler的子类：
public class ServiceInvocationHandler implements InvocationHandler {
private ServiceApi serviceApi;
public ServiceInvocationHandler(ServiceApi serviceApi){
this.serviceApi = serviceApi;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Log.d("TAG","执行了invoke");
//执行传入的serviceApi的method，
return method.invoke(serviceApi,args);
}
}
使用：
//新建ServiceApiImp对象
ServiceApiImp serviceApiImp = new ServiceApiImp();
//新建ServiceInvocationHandler
ServiceInvocationHandler invocationHandler = new ServiceInvocationHandler(serviceApiImp);
//调用Proxy的newProxyInstance方法，传入接口的classLoader和interface类
//还有invocationHandler
//这时候得到的是一个ServiceApi 实例
ServiceApi proxyServiceApi = (ServiceApi) Proxy.newProxyInstance(ServiceApi.class.getClassLoader(), new Class<?>[]{ServiceApi.class},invocationHandler);
//调用它的两个方法
proxyServiceApi.login();
proxyServiceApi.loginOut();
得到打印信息：
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 执行了invoke
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 登录了
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 执行了invoke
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 退出登录了
从效果来看，利用动态代理生成的那个ServiceApi实例在调用自身方法的时候看来实际执行了是传入的那个InvocationHandler里的那个invoke()方法：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Log.d("TAG","执行了invoke");
//执行传入的serviceApi的method，
return method.invoke(serviceApi,args);
}
即invoke()方法既执行了代理的ServiceApi实例的方法，同时也执行了自身的代码，而且这个规则对所有的ServiceApi里的方法都是有效的。
带着这个疑问，我们来看看它是怎么做到的，其实从那个Proxy的newProxyInstance()方法我们可以肯定这里构建了一个ServiceApi的实现类，也就是我们要的代理类，并且new了一个它的实例对象，至于这个实现类长什么样子，我们继续看newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)方法:
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
/*
* Look up or generate the designated proxy class.
查找或者生成指定的代理类
*/
//这个是关键的步骤，注意，就是在这里生成的代理类
Class<?> cl = getProxyClass0(loader, intfs);
try {
//拿到代理类的构造方法，这是一个入参为InvocationHandler的构造方法
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
//判断权限
if (!Modifier.isPublic(cl.getModifiers())) {
cons.setAccessible(true);
}
//最终通过这个构造方法传入InvocationHandler创建了实例
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);
}
}
看到这里，能猜到那个生成的代理类有一个入参为InvocationHandler数组的构造方法，并且就是用这个构造方法创建了实例。
接下来继续追踪那个构建代理类的方法：
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
//翻译上面的注释：如果这个通过实现了给定的接口的给定的classLoader定义的代理类是存
//在的，则会返回一个缓存复制品，否则会通过ProxyClassFactory来创建它
return proxyClassCache.get(loader, interfaces);
}
proxyClassCache是个缓存工具，这里不深究它，直接看看代理类工厂ProxyClassFactory：
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*一个生成，定义，返回给定的classLoader和接口数组的代理类的工厂
*/
//这是Proxy的内部类，主要看apply()方法
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();
//搜集接口名的方法，最后拿去生成类
@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 the default package.
proxyPkg = "";
}
{
// Android-changed: Generate the proxy directly instead of calling
// through to ProxyGenerator.
List<Method> methods = getMethods(interfaces);
Collections.sort(methods, ORDER_BY_SIGNATURE_AND_SUBTYPE);
validateReturnTypes(methods);
List<Class<?>[]> exceptions = deduplicateAndGetExceptions(methods);
Method[] methodsArray = methods.toArray(new Method[methods.size()]);
Class<?>[][] exceptionsArray = exceptions.toArray(new Class<?>[exceptions.size()][]);
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//最终把这些参数丢到这个方法里
return generateProxy(proxyName, interfaces, loader, methodsArray,
exceptionsArray);
}
}
}
//我这个版本是个本地方法，看不到具体实现了- -尴尬了
private static native Class<?> generateProxy(String name, Class<?>[] interfaces,
ClassLoader loader, Method[] methods,
Class<?>[][] exceptions);
很不幸，分析到这里看不下去了，看来只能去网上找找看生成的那个代理类长什么样了，找了一圈发现有很多，结合自己的例子改了改：
// 继承了Proxy类
public final class $Proxy0 extends Proxy implements ServiceApi {
private static Method m1;
private static Method m8;
private static Method m2;
private static Method m3;
private static Method m5;
private static Method m4;
private static Method m7;
private static Method m9;
private static Method m0;
private static Method m6;
//构造函数，入参是InvocationHandler
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
....
....
/**
* 这里是代理类实现的被代理对象的接口的相同方法
*/
public final void login() throws {
try {
// super.h 对应的是父类的h变量，他就是Proxy.nexInstance方法中的InvocationHandler参数
// 所以这里实际上就是使用了我们自己写的InvocationHandler实现类的invoke方法
super.h.invoke(this, m3, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void loginOut() throws {
try {
//可见logOut跟login都是一样的实现
super.h.invoke(this, m3, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final Class getClass() throws {
try {
return (Class)super.h.invoke(this, m7, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
....
....
// 在静态构造块中，代理类通过反射获取了被代理类的详细信息，比如各种方法
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m8 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("notify");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("login");
m4= Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("loginOut");
m5 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("wait", Long.TYPE);
m7 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("getClass");
m9 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("notifyAll");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
m6 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("wait");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
看到这里应该豁然开朗了吧，其实很多听起来牛逼的技术术语，归根结底都是很原始的实现。
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