动态代理源码解析
2019-04-28 本文已影响0人
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//Java 实现
@Proxy.java
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.
*/
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);
}
}
@Proxy.java
//可以看到，实现的接口数量不能大于 65535 的限制
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
return proxyClassCache.get(loader, interfaces);
}
@Proxy.java
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
//来看 Wakcache 的申明<泛型>
final class WeakCache<K, P, V>{
private final BiFunction<K, P, ?> subKeyFactory;
private final BiFunction<K, P, V> valueFactory;
public WeakCache(BiFunction<K, P, ?> subKeyFactory,
BiFunction<K, P, V> valueFactory) {
this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
this.valueFactory = Objects.requireNonNull(valueFactory);
}
}
关键方法
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
//subKeyFacoty在此处是Proxy$KeyFactory，根据 paramter 的个数返回
//Proxy$Key1, Proxy$Key2, Proxy$KeyX
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
//第一次是查询缓存，发现不存在。第二次进来的 suppiler 被赋值为 Factory
V value = supplier.get();
if (value != null) {
return value;
}
}
if (factory == null) {
//Factory 包装了参数，还有 valueFactory
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
supplier = factory;
}
// else retry with winning supplier
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
@WeakCache$Factory
@Override
public synchronized V get() {
Supplier<V> supplier = valuesMap.get(subKey);
if (supplier != this) {
return null;
}
V value = null;
try {
//关键方法，这里的 valueFactory 就是 Proxy$ProxyClassFactory
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) {
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;
}
}
@Proxy$ProxyClassFactory
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) {
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//最最关键
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());
}
}
}
ProxyGenerator.generateProxyClass(final String name,Class[] interfaces)会动态生成一个类的二进制文件，其中类名为 Proxy$name,实现了 interfaces[]接口,附上动态生成的类的文件
interface Goal {
void doSomething();
}
interface Goall {
void doS1();
}
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $proxy1 extends Proxy implements Goal, Goall {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m4;
private static Method m0;
public $proxy1(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 doSomething() 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 void doS1() throws {
try {
super.h.invoke(this, m4, (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("Goal").getMethod("doSomething");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("Goall").getMethod("doS1");
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，生成的类是 Java.lang.reflect.Proxy 的子类，并且实现了传入的接口，重写了 equals,toString,hasCode 方法，因此我们在使用的时候，都是将生成的动态代理下向上转型为希望的接口，然后调用方法。因为 Java 的单继承，多接口，造成了直接动态代理的只能是接口。同时可以看到，在调用接口的方法时，都会转到传入的 InvocationHandler去执行。
在 newProxyInstance()@Proxy 中，根据反射生成这个 Proxy 类，传入 InvocationHandler 作为其构造函数。其他的 Cache 之类，都是对传入的class[]是否是接口之类的权限校验，并且进行缓存。InvocationHandler 内部进行 invoke 的时候已经获得了对应方法的 Method，因此只有 Method 的 invoke 的性能损耗，而没有 findMethod 的性能耗损。
关于动态代理缓存的介绍，参见https://www.cnblogs.com/liuyun1995/p/8144676.html
其关于动态代理类的生成，是严格按照 class 文件的格式进行拼接生成的。
Android 中动态代理的实现与 Java 相比，区别在于 generateProxy( )方法的实现。Java 的generateProxy( )是生成对应的二进制字节码文件，然后加载到 JVM 内存。Android 中，是依赖于虚拟机，譬如在 Dalvik 虚拟机中，
//4.0.4
.//dalvik/vm/native/java_lang_reflect_Proxy.cpp
static void Dalvik_java_lang_reflect_Proxy_generateProxy(const u4* args, JValue* pResult){
StringObject* str = (StringObject*)args[0];
ArrayObject* interfaces = (ArrayObject*) args[1];
Object* loader = (Object*) args[2];
ClassObject* result;
result = dvmGenerateProxyClass(str, interfaces, loader);
RETURN_PTR(result);
}
dvmGenerateProxyClass( )的实现在 Proxy.cpp 中，就是利用虚拟机直接在内存创建一个 Java 层 Class 类在虚拟机对应的 ClassObject 对象，并且为 ClassObject 赋予必要的字段
Dalvik 动态代理的实现
