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java反射之Method的invoke方法实现教程详解

wenyuan65 人气:3
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前言

在框架中经常会会用到method.invoke()方法,用来执行某个的对象的目标方法。以前写代码用到反射时,总是获取先获取Method,然后传入对应的Class实例对象执行方法。然而前段时间研究invoke方法时,发现invoke方法居然包含多态的特性,这是以前没有考虑过的一个问题。那么Method.invoke()方法的执行过程是怎么实现的?它的多态又是如何实现的呢?

本文将从java和JVM的源码实现深入探讨invoke方法的实现过程。

首先给出invoke方法多态特性的演示代码:

public class MethodInvoke {
 
	public static void main(String[] args) throws Exception {
		Method animalMethod = Animal.class.getDeclaredMethod("print");
		Method catMethod = Cat.class.getDeclaredMethod("print");
		
		Animal animal = new Animal();
		Cat cat = new Cat();
		animalMethod.invoke(cat);
		animalMethod.invoke(animal);
		
		catMethod.invoke(cat);
		catMethod.invoke(animal);
	}
	
}
 
class Animal {
	
	public void print() {
		System.out.println("Animal.print()");
	}
	
}
 
class Cat extends Animal {
	
	@Override
	public void print() {
		System.out.println("Cat.print()");
	}
	
}

代码中,Cat类覆盖了父类Animal的print()方法, 然后通过反射分别获取print()的Method对象。最后分别用Cat和Animal的实例对象去执行print()方法。其中animalMethod.invoke(animal)和catMethod.invoke(cat),示例对象的真实类型和Method的声明Classs是相同的,按照预期打印结果;animalMethod.invoke(cat)中,由于Cat是Animal的子类,按照多态的特性,子类调用父类的的方法,方法执行时会动态链接到子类的实现方法上。因此,这里会调用Cat.print()方法;而catMethod.invoke(animal)中,传入的参数类型Animal是父类,却期望调用子类Cat的方法,因此这一次会抛出异常。代码打印结果为:

Cat.print()
Animal.print()
Cat.print()
Exception in thread "main" java.lang.IllegalArgumentException: object is not an instance of declaring class
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
 at java.lang.reflect.Method.invoke(Unknown Source)
 at com.wy.invoke.MethodInvoke.main(MethodInvoke.java:17)

接下来,我们来看看invoke()方法的实现过程。

 public Object invoke(Object obj, Object... args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException
 {
 if (!override) {
 if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
 Class<?> caller = Reflection.getCallerClass(1);
 
 checkAccess(caller, clazz, obj, modifiers);
 }
 }
 MethodAccessor ma = methodAccessor; // read volatile
 if (ma == null) {
 ma = acquireMethodAccessor();
 }
 return ma.invoke(obj, args);
 }

invoke()方法中主要分为两部分:访问控制检查和调用MethodAccessor.invoke()实现方法执行。

首先看一下访问控制检查这一块的逻辑。第一眼看到这里的逻辑的时候,很容易搞不清楚是干嘛的。通俗来讲就是通过方法的修饰符(public/protected/private/package),来判断方法的调用者是否可以访问该方法。这是java的基础内容,不过用代码写出来,一下子不容易想到。访问控制检查分为3步:

  1. 检查override,如果override为true,跳过检查;否则继续;
  2. 快速检查,判断该方法的修饰符modifiers是否为public,如果是跳过检查;否则继续;
  3. 详细检查,通过方法的(protected/private/package)修饰符或方法的声明类(例如子类可以访问父类的protected方法)与调用者caller之间的关系,判断caller是否有权限访问该方法。

override属性是Method的父类AccessibleObject中声明的变量,使得程序可以控制是否跳过访问权限的检查。另外,Method的实例对象中,override属性的初始值设置为false。

 public void setAccessible(boolean flag) throws SecurityException {
 SecurityManager sm = System.getSecurityManager();
 if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 setAccessible0(this, flag);
 }
 
 private static void setAccessible0(AccessibleObject obj, boolean flag)
 throws SecurityException
 {
 if (obj instanceof Constructor && flag == true) {
 Constructor<?> c = (Constructor<?>)obj;
 if (c.getDeclaringClass() == Class.class) {
 throw new SecurityException("Can not make a java.lang.Class" +
   " constructor accessible");
 }
 }
 obj.override = flag;
 }

多说一句,Field同样继承了AccessibleObject,且Field的override也是初始化为false的,也就是说并没有按照变量的修饰符去初始化不同的值。但是我们在调用Field.set(Object obj, Object value)时,如果该Field是private修饰的,会因没有访问权限而抛出异常,因此必须调用setAccessible(true)。此处非常容易理解为因为变量是public的,所以override就被初始化为true。

invoke()方法中,访问控制检查之后,就是通过MethodAccessor.invoke()调用方法。再来看一下代码:

 MethodAccessor ma = methodAccessor; // read volatile
 if (ma == null) {
 ma = acquireMethodAccessor();
 }
 return ma.invoke(obj, args);

这里的逻辑很简单,首先将变量methodAccessor赋值给ma,在方法栈中保存一个可以直接引用的本地变量,如果methodAccessor不存在,调用acquireMethodAccessor()方法创建一个。

 private volatile MethodAccessor methodAccessor;
 private Method root;
 
 private MethodAccessor acquireMethodAccessor() {
 // First check to see if one has been created yet, and take it
 // if so
 MethodAccessor tmp = null;
 if (root != null) tmp = root.getMethodAccessor();
 if (tmp != null) {
 methodAccessor = tmp;
 } else {
 // Otherwise fabricate one and propagate it up to the root
 tmp = reflectionFactory.newMethodAccessor(this);
 setMethodAccessor(tmp);
 }
 
 return tmp;
 }
 
 void setMethodAccessor(MethodAccessor accessor) {
 methodAccessor = accessor;
 // Propagate up
 if (root != null) {
 root.setMethodAccessor(accessor);
 }
 }
 
 Method copy() {
 Method res = new Method(clazz, name, parameterTypes, returnType,
  exceptionTypes, modifiers, slot, signature,
  annotations, parameterAnnotations, annotationDefault);
 res.root = this;
 res.methodAccessor = methodAccessor;
 return res;
 }

综合acquireMethodAccessor(),setMethodAccessor()以及copy()这三个方法,可以看到一个Method实例对象维护了一个root引用。当调用Method.copy()进行方法拷贝时,root指向了被拷贝的对象。那么当一个Method被多次拷贝后,调用一次setMethodAccessor()方法,就会将root引用所指向的Method的methodAccessor变量同样赋值。例如:D -> C -> B -> A,其中X-> Y表示X = Y.copy(), 当C对象调用setMethodAccessor()时,B和A都会传播赋值methodAccessor, 而D的methodAccessor还是null。紧接着,当D需要获取methodAccessor而调用acquireMethodAccessor()时,D获取root的methodAccessor, 那么D将和ABC持有相同的methodAccessor。

虽然Method中,通过维护root引用意图使相同的方法始终保持只有一个methodAccessor实例,但是上述方法仍然无法保证相同的方法只有一个methodAccessor实例。例如通过copy()使ABCD保持关系:D -> C -> B -> A, 当B对象调用setMethodAccessor()时,B和A都会赋值methodAccessor, 而C、D的methodAccessor还是null。这时D调用acquireMethodAccessor()时,D获取root也就是C的methodAccessor,发现为空,然后就新创建了一个。从而出现了相同的方法中出现了两个methodAccessor实例对象的现象。

在Class.getMethod()、Class.getDeclaredMethod()以及Class.getDeclaredMethod(String name, Class<?>... parameterTypes)方法中最终都会调用copy()方法来保障Method使用的安全性。 在比较极端加巧合的情况下,可能会引起类膨胀的问题,这就是接下来要讲到的MethodAccessor的实现机制。

在前面代码中,MethodAccessor的创建是通过反射工厂ReflectionFactory的newMethodAccessor(Method)方法来创建的。

 public MethodAccessor newMethodAccessor(Method method) {
 checkInitted();
 
 if (noInflation) {
 return new MethodAccessorGenerator().
 generateMethod(method.getDeclaringClass(),
  method.getName(),
  method.getParameterTypes(),
  method.getReturnType(),
  method.getExceptionTypes(),
  method.getModifiers());
 } else {
 NativeMethodAccessorImpl acc =
 new NativeMethodAccessorImpl(method);
 DelegatingMethodAccessorImpl res =
 new DelegatingMethodAccessorImpl(acc);
 acc.setParent(res);
 return res;
 }
 }

其中, checkInitted()方法检查从配置项中读取配置并设置noInflation、inflationThreshold的值:

 private static void checkInitted() {
 if (initted) return;
 AccessController.doPrivileged(
 new PrivilegedAction<Void>() {
 public Void run() {
 
  if (System.out == null) {
  // java.lang.System not yet fully initialized
  return null;
  }
 
  String val = System.getProperty("sun.reflect.noInflation");
  if (val != null && val.equals("true")) {
  noInflation = true;
  }
 
  val = System.getProperty("sun.reflect.inflationThreshold");
  if (val != null) {
  try {
  inflationThreshold = Integer.parseInt(val);
  } catch (NumberFormatException e) {
  throw (RuntimeException)
  new RuntimeException("Unable to parse property sun.reflect.inflationThreshold").
   initCause(e);
  }
  }
 
  initted = true;
  return null;
 }
 });
 }

可以通过启动参数-Dsun.reflect.noInflation=false -Dsun.reflect.inflationThreshold=15来设置:

结合字面意思及下面代码理解,这两个配置sun.reflect.noInflation是控制是否立即进行类膨胀,sun.reflect.inflationThreshold是设置类膨胀阈值。

创建MethodAccessor有两种选择,一种是当sun.reflect.noInflation配置项为true时,ReflectionFactory利用MethodAccessor的字节码生成类 MethodAccessorGenerator直接创建一个代理类,通过间接调用原方法完成invoke()任务,具体实现稍后给出。MethodAccessor的另一种实现方式是,创建DelegatingMethodAccessorImpl 委托类,并将执行invoke()方法的具体内容交由NativeMethodAccessorImpl实现,而NativeMethodAccessorImpl最终调用native方法完成invoke()任务。以下是NativeMethodAccessorImpl的invoke()方法实现。

 public Object invoke(Object obj, Object[] args) 
 throws IllegalArgumentException, InvocationTargetException
 {
 if (++numInvocations > ReflectionFactory.inflationThreshold()) {
 MethodAccessorImpl acc = (MethodAccessorImpl)
 new MethodAccessorGenerator().
  generateMethod(method.getDeclaringClass(),
   method.getName(),
   method.getParameterTypes(),
   method.getReturnType(),
   method.getExceptionTypes(),
   method.getModifiers());
 parent.setDelegate(acc);
 }
 
 return invoke0(method, obj, args);
 }
 
 private static native Object invoke0(Method m, Object obj, Object[] args);

可以看到,当numInvocations数量大于配置项sun.reflect.inflationThreshold即类膨胀阈值时, 使用MethodAccessorGenerator创建一个代理类对象,并且将被委托的NativeMethodAccessorImpl的parent,也就是委托类DelegatingMethodAccessorImpl的委托类设置为这个生成的代理对象。这么说可能有点绕,下面用一幅图表示这个过程。

总体来说,当调用invoke()时,按照默认配置,Method首先创建一个DelegatingMethodAccessorImpl对象,并设置一个被委托的NativeMethodAccessorImpl对象,那么method.invoke()就被转换成DelegatingMethodAccessorImpl.invoke(),然后又被委托给NativeMethodAccessorImp.invoke()实现。当NativeMethodAccessorImp.invoke()调用次数超过一定热度时(默认15次),被委托方又被转换成代理类来实现。

之前提到过在极端情况下,同一个方法的Method对象存在多个不同拷贝拷贝时,可能存在多个MethodAccessor对象。那么当多次调用后,必然会生成两个重复功能的代理类。当然,一般情况下,生成两个代理类并没有较大的影响。

其中代理类的具体字节码实现过程较为复杂,大体思想是生成一个如下所示的类:

public class GeneratedMethodAccessor1 extends MethodAccessorImpl {
 
	public GeneratedMethodAccessor1 () {
	 super();
	}
	
	public Object invoke(Object obj, Object[] args)
	 throws IllegalArgumentException, InvocationTargetException 
	{
		if (!(obj instanceof Cat)) {
			throw new ClassCastException();
		}
		if (args != null && args.length != 0) {
			throw new IllegalArgumentException();
		}
		
		try {
			Cat cat = (Cat) obj;
			cat.print();
			return null;
		} catch (Throwable e) {
			throw new InvocationTargetException(e, "invoke error");
		}
	}
	
}

到目前为止,除了在代理的GeneratedMethodAccessor1 类中,方法的执行有多态的特性,而NativeMethodAccessorImp的invoke()实现是在jdk中的完成的。接下来我们将目光移到NativeMethodAccessorImp的native方法invoke0();

openJDK下载地址

首先,在\jdk\src\share\native\sun\reflect路径下找到NativeAccessors.c, 其中有Java_sun_reflect_NativeMethodAccessorImpl _invoke0()方法,根据JNI定义函数名的规则"包名_类名_方法名",这就是我们要找的native方法实现入口。

JNIEXPORT jobject JNICALL Java_sun_reflect_NativeMethodAccessorImpl_invoke0
(JNIEnv *env, jclass unused, jobject m, jobject obj, jobjectArray args)
{
 return JVM_InvokeMethod(env, m, obj, args);
}

方法调用JVM_InvokeMethod(), 一般以JVM_开头的函数定义在jvm.cpp文件中,不熟悉的话可以通过头文件jvm.h看出来。继续追踪,发现jvm.cpp文件位于spot\src\share\vm\prims文件夹下。

JVM_ENTRY(jobject, JVM_InvokeMethod(JNIEnv *env, jobject method, jobject obj, jobjectArray args0))
 JVMWrapper("JVM_InvokeMethod");
 Handle method_handle;
 if (thread->stack_available((address) &method_handle) >= JVMInvokeMethodSlack) {
 method_handle = Handle(THREAD, JNIHandles::resolve(method));
 Handle receiver(THREAD, JNIHandles::resolve(obj));
 objArrayHandle args(THREAD, objArrayOop(JNIHandles::resolve(args0)));
 oop result = Reflection::invoke_method(method_handle(), receiver, args, CHECK_NULL);
 jobject res = JNIHandles::make_local(env, result);
 if (JvmtiExport::should_post_vm_object_alloc()) {
 oop ret_type = java_lang_reflect_Method::return_type(method_handle());
 assert(ret_type != NULL, "sanity check: ret_type oop must not be NULL!");
 if (java_lang_Class::is_primitive(ret_type)) {
 // Only for primitive type vm allocates memory for java object.
 // See box() method.
 JvmtiExport::post_vm_object_alloc(JavaThread::current(), result);
 }
 }
 return res;
 } else {
 THROW_0(vmSymbols::java_lang_StackOverflowError());
 }
JVM_END

其中oop result = Reflection::invoke_method(method_handle(), receiver, args, CHECK_NULL)是方法的执行过程,在\hotspot\src\share\vm\runtime路径下找到reflection.cpp文件。

oop Reflection::invoke_method(oop method_mirror, Handle receiver, objArrayHandle args, TRAPS) {
 oop mirror = java_lang_reflect_Method::clazz(method_mirror);
 int slot = java_lang_reflect_Method::slot(method_mirror);
 bool override = java_lang_reflect_Method::override(method_mirror) != 0;
 objArrayHandle ptypes(THREAD, objArrayOop(java_lang_reflect_Method::parameter_types(method_mirror)));
 
 oop return_type_mirror = java_lang_reflect_Method::return_type(method_mirror);
 BasicType rtype;
 if (java_lang_Class::is_primitive(return_type_mirror)) {
 rtype = basic_type_mirror_to_basic_type(return_type_mirror, CHECK_NULL);
 } else {
 rtype = T_OBJECT;
 }
 
 instanceKlassHandle klass(THREAD, java_lang_Class::as_Klass(mirror));
 Method* m = klass->method_with_idnum(slot);
 if (m == NULL) {
 THROW_MSG_0(vmSymbols::java_lang_InternalError(), "invoke");
 }
 methodHandle method(THREAD, m);
 
 return invoke(klass, method, receiver, override, ptypes, rtype, args, true, THREAD);
}
 
oop Reflection::invoke(instanceKlassHandle klass, methodHandle reflected_method,
  Handle receiver, bool override, objArrayHandle ptypes,
  BasicType rtype, objArrayHandle args, bool is_method_invoke, TRAPS) {
 ResourceMark rm(THREAD);
 
 methodHandle method; // actual method to invoke
 KlassHandle target_klass; // target klass, receiver's klass for non-static
 
 // Ensure klass is initialized
 klass->initialize(CHECK_NULL);
 
 bool is_static = reflected_method->is_static();
 if (is_static) {
 // ignore receiver argument
 method = reflected_method;
 target_klass = klass;
 } else {
 // check for null receiver
 if (receiver.is_null()) {
 THROW_0(vmSymbols::java_lang_NullPointerException());
 }
 // Check class of receiver against class declaring method
 if (!receiver->is_a(klass())) {
 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "object is not an instance of declaring class");
 }
 // target klass is receiver's klass
 target_klass = KlassHandle(THREAD, receiver->klass());
 // no need to resolve if method is private or <init>
 if (reflected_method->is_private() || reflected_method->name() == vmSymbols::object_initializer_name()) {
 method = reflected_method;
 } else {
 // resolve based on the receiver
 if (reflected_method->method_holder()->is_interface()) {
 // resolve interface call
 if (ReflectionWrapResolutionErrors) {
 // new default: 6531596
 // Match resolution errors with those thrown due to reflection inlining
 // Linktime resolution & IllegalAccessCheck already done by Class.getMethod()
 method = resolve_interface_call(klass, reflected_method, target_klass, receiver, THREAD);
 if (HAS_PENDING_EXCEPTION) {
 // Method resolution threw an exception; wrap it in an InvocationTargetException
 oop resolution_exception = PENDING_EXCEPTION;
 CLEAR_PENDING_EXCEPTION;
 JavaCallArguments args(Handle(THREAD, resolution_exception));
 THROW_ARG_0(vmSymbols::java_lang_reflect_InvocationTargetException(),
 vmSymbols::throwable_void_signature(),
 &args);
 }
 } else {
 method = resolve_interface_call(klass, reflected_method, target_klass, receiver, CHECK_(NULL));
 }
 } else {
 // if the method can be overridden, we resolve using the vtable index.
 assert(!reflected_method->has_itable_index(), "");
 int index = reflected_method->vtable_index();
 method = reflected_method;
 if (index != Method::nonvirtual_vtable_index) {
 // target_klass might be an arrayKlassOop but all vtables start at
 // the same place. The cast is to avoid virtual call and assertion.
 InstanceKlass* inst = (InstanceKlass*)target_klass();
 method = methodHandle(THREAD, inst->method_at_vtable(index));
 }
 if (!method.is_null()) {
 // Check for abstract methods as well
 if (method->is_abstract()) {
 // new default: 6531596
 if (ReflectionWrapResolutionErrors) {
 ResourceMark rm(THREAD);
 Handle h_origexception = Exceptions::new_exception(THREAD,
  vmSymbols::java_lang_AbstractMethodError(),
  Method::name_and_sig_as_C_string(target_klass(),
  method->name(),
  method->signature()));
 JavaCallArguments args(h_origexception);
 THROW_ARG_0(vmSymbols::java_lang_reflect_InvocationTargetException(),
 vmSymbols::throwable_void_signature(),
 &args);
 } else {
 ResourceMark rm(THREAD);
 THROW_MSG_0(vmSymbols::java_lang_AbstractMethodError(),
  Method::name_and_sig_as_C_string(target_klass(),
    method->name(),
    method->signature()));
 }
 }
 }
 }
 }
 }
 
 // I believe this is a ShouldNotGetHere case which requires
 // an internal vtable bug. If you ever get this please let Karen know.
 if (method.is_null()) {
 ResourceMark rm(THREAD);
 THROW_MSG_0(vmSymbols::java_lang_NoSuchMethodError(),
 Method::name_and_sig_as_C_string(klass(),
    reflected_method->name(),
    reflected_method->signature()));
 }
 
 // In the JDK 1.4 reflection implementation, the security check is
 // done at the Java level
 if (!(JDK_Version::is_gte_jdk14x_version() && UseNewReflection)) {
 
 // Access checking (unless overridden by Method)
 if (!override) {
 if (!(klass->is_public() && reflected_method->is_public())) {
 bool access = Reflection::reflect_check_access(klass(), reflected_method->access_flags(), target_klass(), is_method_invoke, CHECK_NULL);
 if (!access) {
 return NULL; // exception
 }
 }
 }
 
 } // !(Universe::is_gte_jdk14x_version() && UseNewReflection)
 
 assert(ptypes->is_objArray(), "just checking");
 int args_len = args.is_null() ? 0 : args->length();
 // Check number of arguments
 if (ptypes->length() != args_len) {
 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "wrong number of arguments");
 }
 
 // Create object to contain parameters for the JavaCall
 JavaCallArguments java_args(method->size_of_parameters());
 
 if (!is_static) {
 java_args.push_oop(receiver);
 }
 
 for (int i = 0; i < args_len; i++) {
 oop type_mirror = ptypes->obj_at(i);
 oop arg = args->obj_at(i);
 if (java_lang_Class::is_primitive(type_mirror)) {
 jvalue value;
 BasicType ptype = basic_type_mirror_to_basic_type(type_mirror, CHECK_NULL);
 BasicType atype = unbox_for_primitive(arg, &value, CHECK_NULL);
 if (ptype != atype) {
 widen(&value, atype, ptype, CHECK_NULL);
 }
 switch (ptype) {
 case T_BOOLEAN: java_args.push_int(value.z); break;
 case T_CHAR: java_args.push_int(value.c); break;
 case T_BYTE: java_args.push_int(value.b); break;
 case T_SHORT: java_args.push_int(value.s); break;
 case T_INT: java_args.push_int(value.i); break;
 case T_LONG: java_args.push_long(value.j); break;
 case T_FLOAT: java_args.push_float(value.f); break;
 case T_DOUBLE: java_args.push_double(value.d); break;
 default:
 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "argument type mismatch");
 }
 } else {
 if (arg != NULL) {
 Klass* k = java_lang_Class::as_Klass(type_mirror);
 if (!arg->is_a(k)) {
 THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "argument type mismatch");
 }
 }
 Handle arg_handle(THREAD, arg); // Create handle for argument
 java_args.push_oop(arg_handle); // Push handle
 }
 }
 
 assert(java_args.size_of_parameters() == method->size_of_parameters(), "just checking");
 
 // All oops (including receiver) is passed in as Handles. An potential oop is returned as an
 // oop (i.e., NOT as an handle)
 JavaValue result(rtype);
 JavaCalls::call(&result, method, &java_args, THREAD);
 
 if (HAS_PENDING_EXCEPTION) {
 // Method threw an exception; wrap it in an InvocationTargetException
 oop target_exception = PENDING_EXCEPTION;
 CLEAR_PENDING_EXCEPTION;
 JavaCallArguments args(Handle(THREAD, target_exception));
 THROW_ARG_0(vmSymbols::java_lang_reflect_InvocationTargetException(),
 vmSymbols::throwable_void_signature(),
 &args);
 } else {
 if (rtype == T_BOOLEAN || rtype == T_BYTE || rtype == T_CHAR || rtype == T_SHORT)
 narrow((jvalue*) result.get_value_addr(), rtype, CHECK_NULL);
 return box((jvalue*) result.get_value_addr(), rtype, CHECK_NULL);
 }
}

Reflection::invoke_method()中调用Reflection::invoke(),然后在Reflection::invoke()方法中,当反射调用的方法是接口方法时,调用Reflection::resolve_interface_call(),该方法依赖LinkResolver::resolve_interface_call()来完成方法的动态链接过程,具体实现就不在这里展示。

method = resolve_interface_call(klass, reflected_method, target_klass, receiver, CHECK_(NULL));
methodHandle Reflection::resolve_interface_call(instanceKlassHandle klass, methodHandle method,
      KlassHandle recv_klass, Handle receiver, TRAPS) {
 assert(!method.is_null() , "method should not be null");
 
 CallInfo info;
 Symbol* signature = method->signature();
 Symbol* name = method->name();
 LinkResolver::resolve_interface_call(info, receiver, recv_klass, klass,
     name, signature,
     KlassHandle(), false, true,
     CHECK_(methodHandle()));
 return info.selected_method();
}

如果反射调用的方法是可以被覆盖的方法,例如Animal.print(), Reflection::invoke()最终通过查询虚方法表vtable来确定最终的method。

 // if the method can be overridden, we resolve using the vtable index.
 assert(!reflected_method->has_itable_index(), "");
 int index = reflected_method->vtable_index();
 method = reflected_method;
 if (index != Method::nonvirtual_vtable_index) {
  // target_klass might be an arrayKlassOop but all vtables start at
  // the same place. The cast is to avoid virtual call and assertion.
  InstanceKlass* inst = (InstanceKlass*)target_klass();
  method = methodHandle(THREAD, inst->method_at_vtable(index));
 }

总结

1.method.invoke()方法支持多态特性,其native实现在方法真正执行之前通过动态连接或者虚方法表来实现。

2.框架中使用method.invoke()执行方法调用时,初始获取method对象时,可以先调用一次setAccessable(true),使得后面每次调用invoke()时,节省一次方法修饰符的判断,略微提升性能。业务允许的情况下,Field同样可以如此操作。

3.委托模式可以解决一种方案的多种实现之间自由切换,而代理模式只能根据传入的被代理对象来实现功能。

参考文章:

JAVA深入研究——Method的Invoke方法。

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