JDK1.8里Method.invoke()的实现原理

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我是靠谱客的博主笑点低凉面，这篇文章主要介绍JDK1.8里Method.invoke()的实现原理，现在分享给大家，希望可以做个参考。

Method.invoke()用途广泛
例子

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import java.lang.reflect.Method;

public class MethodTest {
    public static void main(String[] args) throws Exception {
        Class<?> clz = Class.forName("com.method.TestClassLoad");
        Object o = clz.newInstance();
        Method m = clz.getMethod("test", String.class,Integer.class);        
        for (int i = 0; i < 16; i++) {
            m.invoke(o, "huige",i);
        }
    }

}
class TestClassLoad {
    public void test(String s,Integer i){
        System.out.println(s+":测试："+i);
    }
}

注意到MethodTest 类上不会有对类TestClassLoad 的符号依赖——也就是说在加载并初始化MethodTest 类时不需要关心类TestClassLoad 的存在与否，而是等到main()方法执行到调用Class.forName()时才试图对类TestClassLoad 做动态加载；这里用的是一个参数版的forName()，也就是使用当前方法所在类的ClassLoader来加载，并且初始化新加载的类

原理
JDK里Method.invoke()是怎么实现的。
java.lang.reflect.Method#invoke

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

private Method  root;

//获取MethodAccessor
    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;
    }
  //把新创建methodAccessor对象通过root包装起来
  void setMethodAccessor(MethodAccessor accessor) {
        methodAccessor = accessor;
        // Propagate up
        if (root != null) {
            root.setMethodAccessor(accessor);
        }
    }

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

    private Method  root;

    //获取MethodAccessor
    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;
    }
  //把新创建methodAccessor对象通过root包装起来
  void setMethodAccessor(MethodAccessor accessor) {
        methodAccessor = accessor;
        // Propagate up
        if (root != null) {
            root.setMethodAccessor(accessor);
        }
    }

可以看到Method.invoke()实际上并不是自己实现的反射调用逻辑，而是委托给sun.reflect.MethodAccessor来处理。每个实际的Java方法只有一个对应的Method对象作为root。这个root是不会暴露给用户的，而是每次在通过反射获取Method对象时，把新创建methodAccessor对象通过root包装起来。在第一次调用一个实际Java方法对应的Method对象的invoke()方法之前，实现调用逻辑的MethodAccessor对象还没创建；等第一次调用时才新创建MethodAccessor并更新给root，然后调用MethodAccessor.invoke()真正完成反射调用。

那么MethodAccessor是啥呢？
sun.reflect.MethodAccessor

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public interface MethodAccessor {
    Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException;
}
可以看到它只是一个单方法接口，其invoke()方法与Method.invoke()的对应。

创建MethodAccessor实例的是ReflectionFactory。
sun.reflect.ReflectionFactory#newMethodAccessor

private static boolean noInflation = false;
    //调用超过15次就采用java版本
    private static int inflationThreshold = 15;
    private static void checkInitted() {
        if (!initted) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    if (System.out == null) {
                        return null;
                    } else {
                        String var1 = System.getProperty("sun.reflect.noInflation");
                        if (var1 != null && var1.equals("true")) {
                            ReflectionFactory.noInflation = true;
                        }

var1 = System.getProperty("sun.reflect.inflationThreshold");
                        if (var1 != null) {
                            try {
                                ReflectionFactory.inflationThreshold = Integer.parseInt(var1);
                            } catch (NumberFormatException var3) {
                                throw new RuntimeException("Unable to parse property sun.reflect.inflationThreshold", var3);
                            }
                        }

ReflectionFactory.initted = true;
                        return null;
                    }
                }
            });
        }
    }
    public MethodAccessor newMethodAccessor(Method var1) {
        checkInitted();
        if (noInflation && !ReflectUtil.isVMAnonymousClass(var1.getDeclaringClass())) {
            return (new MethodAccessorGenerator()).generateMethod(var1.getDeclaringClass(), var1.getName(), var1.getParameterTypes(), var1.getReturnType(), var1.getExceptionTypes(), var1.getModifiers());
        } else {
            NativeMethodAccessorImpl var2 = new NativeMethodAccessorImpl(var1);
            DelegatingMethodAccessorImpl var3 = new DelegatingMethodAccessorImpl(var2);
            var2.setParent(var3);
            return var3;
        }

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public interface MethodAccessor {
    Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException;
}
可以看到它只是一个单方法接口，其invoke()方法与Method.invoke()的对应。

创建MethodAccessor实例的是ReflectionFactory。
sun.reflect.ReflectionFactory#newMethodAccessor

    private static boolean noInflation = false;
    //调用超过15次就采用java版本
    private static int inflationThreshold = 15;
    private static void checkInitted() {
        if (!initted) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    if (System.out == null) {
                        return null;
                    } else {
                        String var1 = System.getProperty("sun.reflect.noInflation");
                        if (var1 != null && var1.equals("true")) {
                            ReflectionFactory.noInflation = true;
                        }

                        var1 = System.getProperty("sun.reflect.inflationThreshold");
                        if (var1 != null) {
                            try {
                                ReflectionFactory.inflationThreshold = Integer.parseInt(var1);
                            } catch (NumberFormatException var3) {
                                throw new RuntimeException("Unable to parse property sun.reflect.inflationThreshold", var3);
                            }
                        }

                        ReflectionFactory.initted = true;
                        return null;
                    }
                }
            });
        }
    }
    public MethodAccessor newMethodAccessor(Method var1) {
        checkInitted();
        if (noInflation && !ReflectUtil.isVMAnonymousClass(var1.getDeclaringClass())) {
            return (new MethodAccessorGenerator()).generateMethod(var1.getDeclaringClass(), var1.getName(), var1.getParameterTypes(), var1.getReturnType(), var1.getExceptionTypes(), var1.getModifiers());
        } else {
            NativeMethodAccessorImpl var2 = new NativeMethodAccessorImpl(var1);
            DelegatingMethodAccessorImpl var3 = new DelegatingMethodAccessorImpl(var2);
            var2.setParent(var3);
            return var3;
        }

从上面代码可以看出：实际的MethodAccessor实现有两个版本
（1）一个是Java实现的。Java实现的版本在初始化时需要较多时间，但长久来说性能较好；
（2）另一个是native code实现的。native版本正好相反，启动时相对较快，但运行时间长了之后速度就比不过Java版了。这是HotSpot的优化方式带来的性能特性，同时也是许多虚拟机的共同点：跨越native边界会对优化有阻碍作用，它就像个黑箱一样让虚拟机难以分析也将其内联，于是运行时间长了之后反而是托管版本的代码更快些。
为了权衡两个版本的性能，Sun的JDK使用了inflation的技巧：让Java方法在被反射调用时，开头若干次使用native版，等反射调用次数超过阈值（15次）时则生成一个专用的MethodAccessor实现类，生成其中的invoke()方法的字节码，以后对该Java方法的反射调用就会使用Java版。

验证下：
编译上述代码，并在执行MethodTest 时加入-XX:+TraceClassLoading参数（或者-verbose:class或者直接-verbose都行），如下：

java -XX:+TraceClassLoading MethodTest

如果用IDEA，可以通过

点击Edit Configurations配置

image
可以看到输出了一大堆log，把其中相关的部分截取出来如下

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huige:测试：0
huige:测试：1
huige:测试：2
huige:测试：3
huige:测试：4
huige:测试：5
huige:测试：6
huige:测试：7
huige:测试：8
huige:测试：9
huige:测试：10
huige:测试：11
huige:测试：12
huige:测试：13
huige:测试：14
[Loaded sun.reflect.ClassFileConstants from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.AccessorGenerator from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.MethodAccessorGenerator from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVectorFactory from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVector from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVectorImpl from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassFileAssembler from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.UTF8 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.Label from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.Label$PatchInfo from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.MethodAccessorGenerator$1 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassDefiner from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassDefiner$1 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]
huige:测试：15

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huige:测试：0
huige:测试：1
huige:测试：2
huige:测试：3
huige:测试：4
huige:测试：5
huige:测试：6
huige:测试：7
huige:测试：8
huige:测试：9
huige:测试：10
huige:测试：11
huige:测试：12
huige:测试：13
huige:测试：14
[Loaded sun.reflect.ClassFileConstants from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.AccessorGenerator from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.MethodAccessorGenerator from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVectorFactory from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVector from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ByteVectorImpl from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassFileAssembler from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.UTF8 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.Label from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.Label$PatchInfo from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.MethodAccessorGenerator$1 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassDefiner from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.ClassDefiner$1 from D:projectJavajdk1.8.0_101jrelibrt.jar]
[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]
huige:测试：15

可以看到在执行16次时被触发了，导致JVM新加载了一堆类，其中就包括[Loaded sun.reflect.GeneratedMethodAccessor1 from JVM_DefineClass]这么一行

具体实现如下：

（1）MethodAccessor实现版本：开头若干次使用native版
通过DelegatingMethodAccessorImpl实现的，代码如下：

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```java
class DelegatingMethodAccessorImpl extends MethodAccessorImpl {
    private MethodAccessorImpl delegate;

DelegatingMethodAccessorImpl(MethodAccessorImpl var1) {
        this.setDelegate(var1);
    }

public Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException {
        return this.delegate.invoke(var1, var2);
    }

//var1就是NativeMethodAccessorImpl var1 = new NativeMethodAccessorImpl(var1);
    void setDelegate(MethodAccessorImpl var1) {
        this.delegate = var1;
    }
当Method.invoke()调用时，同时调用sun.reflect.DelegatingMethodAccessorImpl#invoke方法，即调用sun.reflect.NativeMethodAccessorImpl#invoke方法，代码如下：

class NativeMethodAccessorImpl extends MethodAccessorImpl {
    private final Method method;
    private DelegatingMethodAccessorImpl parent;
    private int numInvocations;

NativeMethodAccessorImpl(Method var1) {
        this.method = var1;
    }

public Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException {
        if (++this.numInvocations > ReflectionFactory.inflationThreshold() && !ReflectUtil.isVMAnonymousClass(this.method.getDeclaringClass())) {
            MethodAccessorImpl var3 = (MethodAccessorImpl)(new MethodAccessorGenerator()).generateMethod(this.method.getDeclaringClass(), this.method.getName(), this.method.getParameterTypes(), this.method.getReturnType(), this.method.getExceptionTypes(), this.method.getModifiers());
            this.parent.setDelegate(var3);
        }

return invoke0(this.method, var1, var2);
    }

void setParent(DelegatingMethodAccessorImpl var1) {
        this.parent = var1;
    }

private static native Object invoke0(Method var0, Object var1, Object[] var2);
}

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```java
class DelegatingMethodAccessorImpl extends MethodAccessorImpl {
    private MethodAccessorImpl delegate;

    DelegatingMethodAccessorImpl(MethodAccessorImpl var1) {
        this.setDelegate(var1);
    }

    public Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException {
        return this.delegate.invoke(var1, var2);
    }

  //var1就是NativeMethodAccessorImpl var1 = new NativeMethodAccessorImpl(var1);
    void setDelegate(MethodAccessorImpl var1) {
        this.delegate = var1;
    }
当Method.invoke()调用时，同时调用sun.reflect.DelegatingMethodAccessorImpl#invoke方法，即调用sun.reflect.NativeMethodAccessorImpl#invoke方法，代码如下：

class NativeMethodAccessorImpl extends MethodAccessorImpl {
    private final Method method;
    private DelegatingMethodAccessorImpl parent;
    private int numInvocations;

    NativeMethodAccessorImpl(Method var1) {
        this.method = var1;
    }

    public Object invoke(Object var1, Object[] var2) throws IllegalArgumentException, InvocationTargetException {
        if (++this.numInvocations > ReflectionFactory.inflationThreshold() && !ReflectUtil.isVMAnonymousClass(this.method.getDeclaringClass())) {
            MethodAccessorImpl var3 = (MethodAccessorImpl)(new MethodAccessorGenerator()).generateMethod(this.method.getDeclaringClass(), this.method.getName(), this.method.getParameterTypes(), this.method.getReturnType(), this.method.getExceptionTypes(), this.method.getModifiers());
            this.parent.setDelegate(var3);
        }

        return invoke0(this.method, var1, var2);
    }

    void setParent(DelegatingMethodAccessorImpl var1) {
        this.parent = var1;
    }

    private static native Object invoke0(Method var0, Object var1, Object[] var2);
}

从上面代码可以看出：每次NativeMethodAccessorImpl.invoke()方法被调用时，都会增加一个调用次数计数器numInvocations，看超过阈值没有；一旦超过，则调用MethodAccessorGenerator.generateMethod()来生成Java版的MethodAccessor的实现类，并且改变DelegatingMethodAccessorImpl所引用的MethodAccessor为Java版。后续经由DelegatingMethodAccessorImpl.invoke()调用到的就是Java版的实现了。

注意到关键的invoke0()方法是个native方法。它在HotSpot VM里是由JVM_InvokeMethod()函数所支持的：

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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);  
}  
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  
其中的关键又是Reflection::invoke_method()：

// This would be nicer if, say, java.lang.reflect.Method was a subclass  
// of java.lang.reflect.Constructor  
  
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_klassOop(mirror));  
  methodOop 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);  
}

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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);  
}  
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  
其中的关键又是Reflection::invoke_method()：

// This would be nicer if, say, java.lang.reflect.Method was a subclass  
// of java.lang.reflect.Constructor  
  
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_klassOop(mirror));  
  methodOop 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);  
}

（2）MethodAccessor实现版本：java版本MethodAccessorGenerator
MethodAccessorGenerato如何实现的呢？
具体实现在sun.reflect.MethodAccessorGenerator#generate方法
代码如下：

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private MagicAccessorImpl generate(final Class<?> var1, String var2, Class<?>[] var3, Class<?> var4, Class<?>[] var5, int var6, boolean var7, boolean var8, Class<?> var9) {
        //......
      //这里生成[Loaded sun.reflect.GeneratedMethodAccessor1 from __JVM_DefineClass__]
        final String var13 = generateName(var7, var8);
        //。。。。。。
        this.targetMethodRef = this.asm.cpi();
        if (var7) {
            //构建newInstance
            this.asm.emitConstantPoolUTF8("newInstance");
        } else {
           //构建invoke
            this.asm.emitConstantPoolUTF8("invoke");
        }
      //.........
    }

最后生成的Java版MethodAccessor大致如下

public class GeneratedMethodAccessor1 extends MethodAccessorImpl {
public GeneratedMethodAccessor1() {
super();
}

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public Object invoke(Object obj, Object[] args)     
    throws IllegalArgumentException, InvocationTargetException {  
    // prepare the target and parameters  
    if (obj == null) throw new NullPointerException();  
    try {  
        TestClassLoad target = (TestClassLoad) obj;             
        String arg0 = (String) args[0];  
        Integer arg1 = (Integer ) args[1];  
    } catch (ClassCastException e) {  
        throw new IllegalArgumentException(e.toString());  
    } catch (NullPointerException e) {  
        throw new IllegalArgumentException(e.toString());  
    }  
    // 调用目标方法
    try {  
        target.test(arg0,arg1 );  
    } catch (Throwable t) {  
        throw new InvocationTargetException(t);  
    }  
}

}

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总结
就反射调用而言，这个invoke()方法非常干净（然而就“正常调用”而言这额外开销还是明显的）。注意到参数数组被拆开了，把每个参数都恢复到原本没有被Object[]包装前的样子，然后对目标方法做正常的invokevirtual调用。由于在生成代码时已经循环遍历过参数类型的数组，生成出来的代码里就不再包含循环了。
当该反射调用成为热点时，它甚至可以被内联到靠近Method.invoke()的一侧，大大降低了反射调用的开销。而native版的反射调用则无法被有效内联，因而调用开销无法随程序的运行而降低。
虽说Sun的JDK这种实现方式使得反射调用方法成本比以前降低了很多，但Method.invoke()本身要用数组包装参数；而且每次调用都必须检查方法的可见性（在Method.invoke()里），也必须检查每个实际参数与形式参数的类型匹配性（在NativeMethodAccessorImpl.invoke0()里或者生成的Java版MethodAccessor.invoke()里）；而且Method.invoke()就像是个独木桥一样，各处的反射调用都要挤过去，在调用点上收集到的类型信息就会很乱，影响内联程序的判断，使得Method.invoke()自身难以被内联到调用方。
相比之下MethodHandle则更有潜力，在其功能完全实现后能达到比普通反射调用方法更高的性能。在使用MethodHandle来做反射调用时，MethodHandle.invoke()的形式参数与返回值类型都是准确的，所以只需要在链接方法的时候才需要检查类型的匹配性，而不必在每次调用时都检查。而且MethodHandle是不可变值，在创建后其内部状态就不会再改变了；JVM可以利用这个知识而放心的对它做激进优化，例如将实际的调用目标内联到做反射调用的一侧