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简单Demo
我们知道Java中提供了动态代理机制，可以动态进行代理，示例代码如下：
public class ProxyFactory {
private Object target;
public ProxyFactory(Object target) {
this.target = target;
}
// 生成动态代理对象
public Object getProxyInstance() {
return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(),
new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("开始调用真正的方法");
// 调用被代理对象的方法
Object returnValue = method.invoke(target, args);
System.out.println("结束调用真正的方法");
return null;
}
});
}
}
复制代码那么，动态代理在Android的ART中是怎么实现的呢？
下面通过阅读代码简单探究一下。
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源码
2.1 Proxy.newProxyInstance [Proxy.java]
@CallerSensitive
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.
*/
// 动态创建一个Proxy的子类，见2.1.1
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
cons.setAccessible(true);
}
// 初始化子类，并将InvocationHandler传递过去
// 上面创建的Proxy子类中，已经将要代理的method的入口地址设置为了
// InvocationHandler.invoke()方法的入口
// 因此在调用cl的方法时就会调用到h.invoke()方法
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);
}
}
复制代码2.1.1 Proxy.getProxyClass0 [Proxy.java]
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
// 如果在缓存中可以找到，直接返回
// 如果无法找到，调用ProxyClassFactory.apply()方法动态生成，
// 具体原因见2.1.1.1
return proxyClassCache.get(loader, interfaces);
}
复制代码2.1.1.1 init WeakCache
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
复制代码2.1.2 Proxy.ProxyClassFactory.apply [Proxy.java]
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
// 判断interfaces里面是否有重复的，有重复抛出异常
// 代码省略....
// 判断是否non public interfaces都在同一个package中，如果不在，抛出异常
// 代码省略....
{
// Android-changed: Generate the proxy directly instead of calling
// through to ProxyGenerator.
// 获取所有interface的声明方法
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;
// 生成proxy对象
// 这是一个native方法
return generateProxy(proxyName, interfaces, loader, methodsArray,
exceptionsArray);
}
}
}
复制代码2.1.3 Proxy_generateProxy [java_lang_reflect_Proxy.cc]
static jclass Proxy_generateProxy(JNIEnv* env, jclass, jstring name, jobjectArray interfaces,
jobject loader, jobjectArray methods, jobjectArray throws) {
ScopedFastNativeObjectAccess soa(env);
ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
return soa.AddLocalReference<jclass>(class_linker->CreateProxyClass(
soa, name, interfaces, loader, methods, throws));
}
复制代码2.1.4 ClassLinker::CreateProxyClass [class_linker.cc]
ObjPtr<mirror::Class> ClassLinker::CreateProxyClass(ScopedObjectAccessAlreadyRunnable& soa,
jstring name,
jobjectArray interfaces,
jobject loader,
jobjectArray methods,
jobjectArray throws) {
Thread* self = soa.Self();
//...
StackHandleScope<12> hs(self);
// 初始化一个temp_kcass
MutableHandle<mirror::Class> temp_klass(hs.NewHandle(
AllocClass(self, GetClassRoot<mirror::Class>(this), sizeof(mirror::Class))));
if (temp_klass == nullptr) {
CHECK(self->IsExceptionPending()); // OOME.
return nullptr;
}
// 给设置一堆标记位
// ...
// Proxies have 1 direct method, the constructor
const size_t num_direct_methods = 1;
// List of the actual virtual methods this class will have.
std::vector<ArtMethod*> proxied_methods;
// 这里会有一堆逻辑进行一下过滤，把private、final、static方法给过滤掉
// 只留下virtual_methods，可以进行代理
// ...
const size_t num_virtual_methods = proxied_methods.size();
// 处理throw methods
// ...
temp_klass->SetMethodsPtr(proxy_class_methods, num_direct_methods, num_virtual_methods);
// Create the single direct method.
CreateProxyConstructor(temp_klass, temp_klass->GetDirectMethodUnchecked(0, image_pointer_size_));
// Create virtual method using specified prototypes.
// TODO These should really use the iterators.
for (size_t i = 0; i < num_virtual_methods; ++i) {
auto* virtual_method = temp_klass->GetVirtualMethodUnchecked(i, image_pointer_size_);
auto* prototype = proxied_methods[i];
// 重点在这里，这里面会进行方法代理，见2.1.5
CreateProxyMethod(temp_klass, prototype, virtual_method);
DCHECK(virtual_method->GetDeclaringClass() != nullptr);
DCHECK(prototype->GetDeclaringClass() != nullptr);
}
// 把父类设置为java.lang.reflect.Proxy
temp_klass->SetSuperClass(GetClassRoot<mirror::Proxy>(this));
// Now effectively in the loaded state.
mirror::Class::SetStatus(temp_klass, ClassStatus::kLoaded, self);
self->AssertNoPendingException();
// At this point the class is loaded. Publish a ClassLoad event.
// Note: this may be a temporary class. It is a listener's responsibility to handle this.
Runtime::Current()->GetRuntimeCallbacks()->ClassLoad(temp_klass);
MutableHandle<mirror::Class> klass = hs.NewHandle<mirror::Class>(nullptr);
{
// Must hold lock on object when resolved.
ObjectLock<mirror::Class> resolution_lock(self, temp_klass);
// Link the fields and virtual methods, creating vtable and iftables.
// The new class will replace the old one in the class table.
Handle<mirror::ObjectArray<mirror::Class>> h_interfaces(
hs.NewHandle(soa.Decode<mirror::ObjectArray<mirror::Class>>(interfaces)));
// 这里进行link，创建vtable和iftables
if (!LinkClass(self, descriptor, temp_klass, h_interfaces, &klass)) {
if (!temp_klass->IsErroneous()) {
mirror::Class::SetStatus(temp_klass, ClassStatus::kErrorUnresolved, self);
}
return nullptr;
}
}
Runtime::Current()->GetRuntimeCallbacks()->ClassPrepare(temp_klass, klass);
VisiblyInitializedCallback* callback = nullptr;
{
// Lock on klass is released. Lock new class object.
ObjectLock<mirror::Class> initialization_lock(self, klass);
// Conservatively go through the ClassStatus::kInitialized state.
callback = MarkClassInitialized(self, klass);
}
if (callback != nullptr) {
callback->MakeVisible(self);
}
return klass.Get();
}
复制代码2.1.5 ClassLinker::CreateProxyMethod [class_linker.cc]
void ClassLinker::CreateProxyMethod(Handle<mirror::Class> klass, ArtMethod* prototype,
ArtMethod* out) {
// We steal everything from the prototype (such as DexCache, invoke stub, etc.) then specialize
// as necessary
DCHECK(out != nullptr);
out->CopyFrom(prototype, image_pointer_size_);
// Set class to be the concrete proxy class.
out->SetDeclaringClass(klass.Get());
// 设置一堆标记位
// ...
// Set the original interface method.
out->SetDataPtrSize(prototype, image_pointer_size_);
// At runtime the method looks like a reference and argument saving method, clone the code
// related parameters from this method.
// 将方法的入口设置为InvokeHandler的调用入口
out->SetEntryPointFromQuickCompiledCode(GetQuickProxyInvokeHandler());
}
复制代码2.1.5.1 GetQuickproxyInvokeHandler [runtime_asm_entrypoints.h]
extern "C" void art_quick_proxy_invoke_handler();
static inline const void* GetQuickProxyInvokeHandler() {
// 返回了art_quick_proxy_invoke_handler函数指针
// art_quick_proxy_invoke_handler是一个汇编写的函数，以arm64为例，见2.1.5.2
return reinterpret_cast<const void*>(art_quick_proxy_invoke_handler);
}
复制代码2.1.5.2 art_quick_proxy_invoke_handler [quick_entrypoints_arm64.S]
// x0寄存器保存方法名
// x1寄存器保存了上面创建的Poxy子类
ENTRY art_quick_proxy_invoke_handler
SETUP_SAVE_REFS_AND_ARGS_FRAME_WITH_METHOD_IN_X0
mov x2, xSELF // pass Thread::Current
mov x3, sp // pass SP
// 调用artQuickProxyInvokeHandler(Method *proxy_method, receiver, Thread*, SP)
// artQuickProxyInvokeHandler的作用是调用Proxy的invoke方法
// 见2.1.5.3
bl artQuickProxyInvokeHandler // (Method* proxy method, receiver, Thread*, SP)
ldr x2, [xSELF, THREAD_EXCEPTION_OFFSET]
cbnz x2, .Lexception_in_proxy // success if no exception is pending
.cfi_remember_state
RESTORE_SAVE_REFS_AND_ARGS_FRAME // Restore frame
REFRESH_MARKING_REGISTER
fmov d0, x0 // Store result in d0 in case it was float or double
ret // return on success
.Lexception_in_proxy:
CFI_RESTORE_STATE_AND_DEF_CFA sp, FRAME_SIZE_SAVE_REFS_AND_ARGS
RESTORE_SAVE_REFS_AND_ARGS_FRAME
DELIVER_PENDING_EXCEPTION
END art_quick_proxy_invoke_handler
复制代码2.1.5.3 artQuickProxyInvokeHandler [quick-trampoline_entrypoints.cc]
extern "C" uint64_t artQuickProxyInvokeHandler(
ArtMethod* proxy_method, mirror::Object* receiver, Thread* self, ArtMethod** sp)
REQUIRES_SHARED(Locks::mutator_lock_) {
//...
// 调用InvocationHandler的方法
// 见 2.1.5.4
JValue result = InvokeProxyInvocationHandler(soa, shorty, rcvr_jobj, interface_method_jobj, args);
if (soa.Self()->IsExceptionPending()) {
if (instr->HasMethodUnwindListeners()) {
instr->MethodUnwindEvent(self,
proxy_method,
0);
}
} else if (instr->HasMethodExitListeners()) {
instr->MethodExitEvent(self,
proxy_method,
{},
result);
}
return result.GetJ();
}
复制代码2.1.5.4 InvokeProxyInvocationHandler [entrypoint_utils.cc]
JValue InvokeProxyInvocationHandler(ScopedObjectAccessAlreadyRunnable& soa,
const char* shorty,
jobject rcvr_jobj,
jobject interface_method_jobj,
std::vector<jvalue>& args) {
//...
// Call Proxy.invoke(Proxy proxy, Method method, Object[] args).
jvalue invocation_args[3];
invocation_args[0].l = rcvr_jobj;
invocation_args[1].l = interface_method_jobj;
invocation_args[2].l = args_jobj;
// 在这里调用Java层的Proxy.invoke(Proxy proxy, Method method, Object[] args)
// 见2.1.5.5
jobject result =
soa.Env()->CallStaticObjectMethodA(WellKnownClasses::java_lang_reflect_Proxy,
WellKnownClasses::java_lang_reflect_Proxy_invoke,
invocation_args);
//...
}
复制代码2.1.5.5 Proxy.invoke [Proxy.java]
private static Object invoke(Proxy proxy, Method method, Object[] args) throws Throwable {
// 转了一圈，最终回到了这里，通过这里调用了InvocationHandler.invoke()方法
InvocationHandler h = proxy.h;
return h.invoke(proxy, method, args);
}
复制代码-
总结
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Proxy.newProxyInstance() 方法最终会通过调用generateProxy() native方法通过ClassLinker来创建一个实现了传入的interface的java.lang.reflect.Proxy的子类并返回
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创建的Proxy子类会将method的入口设置为调用Proxy.invoke() 方法入口，最终在Proxy.invoke中调用了proxy的InvocationHandler实例属性的invoke() 方法，最终调用到自己实现的invoke方法
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因此在调用newProxyInstance() 返回的子类的方法时，会调用到传入的InvocationHandler.invoke() 方法，然后在invoke方法中进行代理方法的实现
