【總結(jié)】：JDK動(dòng)態(tài)代理的原理是通過(guò)目標(biāo)對(duì)象提供的classloader、interfaces的Class對(duì)象數(shù)組、InvocatiionHandler提供的代理邏輯來(lái)重新定義并在內(nèi)存生成一個(gè)代理類的class文件并且通過(guò)構(gòu)造器對(duì)象實(shí)例化并返回。使得用戶使用代理類對(duì)象調(diào)用業(yè)務(wù)接口方法的時(shí)候，調(diào)用的是invocationHandler的invoke方法，最終完成代理。
- 都很熟悉的以下這段代碼，JDK的動(dòng)態(tài)代理，使用
Proxy.newProxyInstance()獲得代理對(duì)象，并且提供InvocationHandler來(lái)設(shè)置前置和后置處理的相關(guān)代碼。今天看看它的源碼
public interface ISinger {
void sing();
}
public class Singer implements ISinger {
@Override
public void sing() {
System.out.println("唱一會(huì)兒歌");
}
}
public class Test {
public static void main(String[] args) {
Singer target = new Singer();
Object proxyInstance = Proxy.newProxyInstance(
target.getClass().getClassLoader(),
target.getClass().getInterfaces(),
(proxy, method, args1) -> {
System.out.println("代理前：向觀眾問(wèn)好");
Object returnValue = method.invoke(target, args1);
System.out.println("代理后：向觀眾問(wèn)好");
return returnValue;
});
ISinger singerProxy = (ISinger) proxyInstance;
singerProxy.sing();
}
}
newProxyInstance主要做了三件事情，
- 生產(chǎn)制定的代理類
- 獲取構(gòu)造器對(duì)象
- 使用構(gòu)造器對(duì)象實(shí)例化代理對(duì)象
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//判空
Objects.requireNonNull(h);
//克隆該類實(shí)現(xiàn)的所有接口
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.
* 查找或生成特定的代理類
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
* 使用制定的 invocation handler 調(diào)用構(gòu)造器
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
/**
* 常量 constructorParams：
* private static final Class<?>[] constructorParams = { InvocationHandler.class };
**/
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;
}
});
}
// 通過(guò)構(gòu)造器對(duì)象來(lái)實(shí)例化代理對(duì)象并返回
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);
}
}
getProxyClass0()具體負(fù)責(zé)生成代理類
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
//如果緩存中有代理類則使用，如果沒(méi)有則使用ProxyClassFactory生成
return proxyClassCache.get(loader, interfaces);
}
ProxyClassFactory.apply()完成了代理對(duì)象字節(jié)碼的生成
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
//此處省略了一大堆代碼……（主要是接口、包和其他一些類文件相關(guān)的校驗(yàn)）
/*
* Generate the specified proxy class.
* 關(guān)鍵在這里，生成了代理對(duì)象的字節(jié)碼
*/
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());
}
}
}
由于jdk8的源碼到上面的部分就到了native方法了，因此以下內(nèi)容參考了其他博主的內(nèi)容[1]進(jìn)行學(xué)習(xí)
public static byte[] generateProxyClass(final String name, Class<?>[] interfaces, int accessFlags) {
ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
// 真正生成字節(jié)碼的方法
final byte[] classFile = gen.generateClassFile();
// 如果saveGeneratedFiles為true 則生成字節(jié)碼文件，所以在開(kāi)始我們要設(shè)置這個(gè)參數(shù)
// 當(dāng)然，也可以通過(guò)返回的bytes自己輸出
if (saveGeneratedFiles) {
java.security.AccessController.doPrivileged( new java.security.PrivilegedAction<Void>() {
public Void run() {
try {
int i = name.lastIndexOf('.');
Path path;
if (i > 0) {
Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
Files.createDirectories(dir);
path = dir.resolve(name.substring(i+1, name.length()) + ".class");
} else {
path = Paths.get(name + ".class");
}
Files.write(path, classFile);
return null;
} catch (IOException e) {
throw new InternalError( "I/O exception saving generated file: " + e);
}
}
});
}
return classFile;
}
閱讀參考文獻(xiàn)法發(fā)現(xiàn)生成的代理對(duì)象是繼承了Proxy類并且會(huì)實(shí)現(xiàn)用戶定義的接口，這里是ISinger接口。因此獲得代理對(duì)象之后可以進(jìn)行強(qiáng)轉(zhuǎn)并且進(jìn)行調(diào)用。
參考文獻(xiàn)：
