如下List-1，动态代理生成的object是代理类，List-1的执行结果是List-2所示
List-1
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
Object object = Proxy.newProxyInstance(classLoader, new Class[]{EchoService.class}, new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (EchoService.class.isAssignableFrom(method.getDeclaringClass())) {
return new StaticProxyEchoService(new DefaultEchoServiceImpl()).echo((String) args[0]);
}
return null;
}
});
EchoService echoService = (EchoService) object;
echoService.echo("Hello world");
LOG.info(object.getClass().getSuperclass() == Proxy.class);
Class<?>[] interfaces = object.getClass().getInterfaces();
LOG.info(interfaces.length == 1);
LOG.info(interfaces[0] == EchoService.class);
Constructor<?>[] constructors = object.getClass().getConstructors();
LOG.info(constructors.length == 1);
LOG.info(constructors[0].getParameterCount() == 1);
LOG.info(constructors[0].getParameterTypes()[0] == InvocationHandler.class);
List-2
2020-12-22 21:13:46.617 [main] INFO com.mjduan.project.log.LOG info() 27 用时:0
2020-12-22 21:13:46.622 [main] INFO com.mjduan.project.log.LOG info() 27 true
2020-12-22 21:13:46.622 [main] INFO com.mjduan.project.log.LOG info() 27 true
2020-12-22 21:13:46.622 [main] INFO com.mjduan.project.log.LOG info() 27 true
2020-12-22 21:13:46.622 [main] INFO com.mjduan.project.log.LOG info() 27 true
2020-12-22 21:13:46.623 [main] INFO com.mjduan.project.log.LOG info() 27 true
2020-12-22 21:13:46.623 [main] INFO com.mjduan.project.log.LOG info() 27 true
我们可以知道object.getClass的类结构大概是如下List-3
List-3
class $Proxy0 extends Proxy implements EchoService{
$Proxy0(InvocationHandler handler){
super(handler);
}
}
即动态生成的类object继承了Proxy，同时实现了EchoService接口，为什么继承要继承Proxy，这是因为我们自定义的InvocationHandler类实现时赋值给Proxy的属性的，如下List-4是Proxy的构造方法
List-4
protected InvocationHandler h;
private Proxy() {
}
protected Proxy(InvocationHandler h) {
Objects.requireNonNull(h);
this.h = h;
}
Proxy.newProxyInstance()的时候，如下List-5所示
1处要对传入的interfaces进行clone()，因为是数组，会被修改
2处是重点，是具体生成$Proxy的class的地方
3处获取$Proxy的构造方法，使用反射实例化
List-5
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();//1
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);//2
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams);//3
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});//4
} 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);
}
}
Proxy的getProxyClass0方法如下List-6，通过WeakCache来生成class类，
List-6
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);
}
proxyClassCache的定义如下List-7所示，ProxyClassFactory是生成动态代理类的工厂
List-7
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 };
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
...
如下List-8是ProxyClassFactory类的定义，会调用apply方法，这个方法里面
- 1处需要重新加载下接口，为什么：这是因为已经加载了这些接口类不一定是传入的ClassLoader来加载的，Class.forname可以加载类，也可以控制是否初始化，这里参数是false所以不会初始化，ClassLoader.loadClass也可以加载类，但是它俩是有区别的。
- 2处生成动态代理类的完全路径名称
- 3处生成字节数组，ProxyGenerator.generateProxyClass__是sun包下面的
- 4处将字节数组生成类
List-8
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);//1
} 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;//2
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);//3
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);//4
} 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());
}
}
}
字节码生成的$Proxy类里面，接口实现上直接调用了invocationHandler.invoke方法，见https://mp.weixin.qq.com/s/gDO6t89gqgcAglA2jkdwVQ
