深入分析JDK动态代理为什么只能使用接口
初学JDK代理时，我们只知道调用一段构造方法Proxy.newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h)，传入接口，就能自动调用到我们实现的InvocationHandler的invoke方法中。
那么为什么只能使用接口和怎么调用到InvocationHandler的invoke方法中的呢？下面来分析下
- 首先从
Proxy.newProxyInstance看起
@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.
*/
//生成增强之后的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);
}
}
- 调用getProxyClass0
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是Proxy的静态变量，是WeakCache类，里面封装了两个类KeyFactory、ProxyClassFactory，都是BiFunction函数式接口，作转换用
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
- 关注
ProxyClassFactory#apply方法，其内调用了ProxyGenerator#generateProxyClass方法，利用asm字节码重组，生成了代理的字节文件，再调用defineClass0native方法，加载到jvm中。
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.
*/
//关注这里，这里生成了增强的Bean字节码文件
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//调用native方法加载到内存
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());
}
}
}
那么，生成的文件内容是什么呢？我们模拟一下，生成到我们的磁盘中
//偷懒，直接在网上找到的 @link https://www.cnblogs.com/jhxxb/p/10557738.html
import sun.misc.ProxyGenerator;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
interface Dao {
void save();
}
class DaoImpl implements Dao {
@Override
public void save() {
System.out.println("save...");
}
}
===========================
public class DynamicProxyTest {
/**
* 保存 JDK 动态代理生产的类
* @param filePath 保存路径，默认在项目路径下生成 $Proxy0.class 文件
*/
private static void saveProxyFile(String... filePath) {
if (filePath.length != 0) {
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
} else {
FileOutputStream out = null;
try {
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", DaoImpl.class.getInterfaces());
out = new FileOutputStream(filePath[0] + "$Proxy0.class");
out.write(classFile);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (out != null) {
out.flush();
out.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) throws Exception {
saveProxyFile();
Object target = new DaoImpl();
/**
* loader：业务对象的类加载器
* interfaces：业务对象实现的所有接口
* public static Class getProxyClass(ClassLoader loader, Class... interfaces)
*/
Class<?> proxyClass = Proxy.getProxyClass(DaoImpl.class.getClassLoader(), DaoImpl.class.getInterfaces());
InvocationHandler handler = new InvocationHandler() {
/**
* @param proxy 代理对象
* @param method 代理的方法对象
* @param args 方法调用时参数
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Object result = null;
if (method.getName().equals("save")) {
System.out.println("before...");
result = method.invoke(target, args);
System.out.println("after...");
}
return result;
}
};
Dao userDao = (Dao) proxyClass.getConstructor(InvocationHandler.class).newInstance(handler);
userDao.save();
}
}
看到生成的文件，反编译之后
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
final class $Proxy0 extends Proxy implements Dao {
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 save() 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("javabase.dynamic.Dao").getMethod("save");
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());
}
}
}
可以看到，生成的代理类居然继承了Proxy，我们知道java是单继承的，所以JDK动态代理只能代理接口。
在调用save方法时候，会调用 super.h.invoke(this, m3, (Object[])null);由下面的静态代码块可知，m3是我们接口实现的原生方法，而h就是我们实现的InvocationHandler
所以会回调到我们实现的InvocationHandler#invoke
至此，JDK动态代理只能使用接口大致也说清楚了~~