java反射机制之Method invoke执行调用方法例子

原文：java反射机制之Method invoke执行调用方法例子

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昨天在群里跟大家讨论了下java反射调用可变参数的问题，这个问题起因是我们需要反射调用另一个部门提供的方法，我同事说java不能反射调用可变参数的方法，于是我写了个demo证明了他这个观点的错误。但是测试过程中，有一点我不明白，就是反射调用可变参数的方法时，为什么一定要保证传入的参数数组长度为1，在群里跟大家讨论了很多，没有得到确切的答案，参照网上大牛写的东西和我自己跟源码的过程，记录如下：

1.两个类，一个父类，一个子类

package com.reflect.test;public class BaseObject {public void getObjectName(){System.out.println("BaseObject");}}

package com.reflect.test;public class SubObject extends BaseObject{@Overridepublic void getObjectName() {System.out.println("SubObject");}public void getParamsLength(String...params){System.out.println("param's length is:"+params.length);}public void getParamsLength(String param1,String param2){System.out.println(param1 + "-" + param2);}}

2.测试类，主要测试重载方法的调用、可变参数方法的调用、定参方法的调用

package com.reflect.test;import java.lang.reflect.Method;public class ReflectTest {private static final String BASE_OBJECT_PATH = "com.reflect.test.BaseObject";private static final String SUB_OBJECT_PATH = "com.reflect.test.SubObject";public static void main(String[] args) throws Exception{Class<?> bClazz = Class.forName(BASE_OBJECT_PATH);Class<?> sClazz = Class.forName(SUB_OBJECT_PATH);Object bObj = bClazz.newInstance();//父类实例Object sObj = sClazz.newInstance();//子类实例//1.反射调用子类父类的重载方法//多态+动态绑定Method bMethod = bClazz.getDeclaredMethod("getObjectName");bMethod.invoke(bObj);//父类的bMethod调用父类的getObjectName()bMethod.invoke(sObj);//父类的bMethod调用子类的getObjectName();Method sMethod = sClazz.getDeclaredMethod("getObjectName");//不符合多态和动态绑定//sMethod.invoke(bObj);//sMethod调用父类的getObjectName()，会报错：java.lang.IllegalArgumentException: object is not an instance of declaring classsMethod.invoke(sObj);//2.反射调用可变参数的方法Method changeMethod = sClazz.getDeclaredMethod("getParamsLength", String[].class);//可变参数必须这样封装，因为java反射内部实现做了参数个数为1的判断，如果参数长度不为1，则会抛出异常String[] strParams = {"a","b","c"};Object[] cParams = {strParams};changeMethod.invoke(sObj, cParams);//3.反射调用固定长度参数的方法Method unChangeMethod1 = sClazz.getDeclaredMethod("getParamsLength", String.class,String.class);unChangeMethod1.invoke(sObj, "Hello","Java");//也可以写成这样Class<?>[] clazzs = {String.class,String.class};Method unChangeMethod2 = sClazz.getDeclaredMethod("getParamsLength", clazzs);unChangeMethod2.invoke(sObj, "Hello","Java");//下面的这种调用形式也是可以的，不过会报警告//String[] params1 = {"Hello","Java"};//unChangeMethod1.invoke(sObj, params1);}}

下面是JDK里面Method 的invoke方法的源码

从代码中可以看出，先检查 AccessibleObject的override属性是否为true（override属性默认为false）。AccessibleObject是Method,Field,Constructor的父类,可调用setAccessible方法改变，如果设置为true,则表示可以忽略访问权限的限制，直接调用。

如果不是ture,则要进行访问权限检测。用Reflection的quickCheckMemberAccess方法先检查是不是public的，如果不是再用Reflection.getCallerClass()方法获得到调用这个方法的Class,然后做是否有权限访问的校验，校验之后缓存一次，以便下次如果还是这个类来调用就不用去做校验了，直接用上次的结果。

 @CallerSensitive    public Object invoke(Object obj, Object... args)        throws IllegalAccessException, IllegalArgumentException,           InvocationTargetException    {        if (!override) {            if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {                // Until there is hotspot @CallerSensitive support                // can't call Reflection.getCallerClass() here                // Workaround for now: add a frame getCallerClass to                // make the caller at stack depth 2                Class<?> caller = getCallerClass();                checkAccess(caller, clazz, obj, modifiers);            }        }        MethodAccessor ma = methodAccessor;             // read volatile        if (ma == null) {            ma = acquireMethodAccessor();        }        return ma.invoke(obj, args);    }//验证的代码，securityCheckCache就是JDK做的缓存 volatile Object securityCheckCache;    void checkAccess(Class<?> caller, Class<?> clazz, Object obj, int modifiers)        throws IllegalAccessException    {        if (caller == clazz) {  // quick check            return;             // ACCESS IS OK        }        Object cache = securityCheckCache;  // read volatile        Class<?> targetClass = clazz;        if (obj != null            && Modifier.isProtected(modifiers)            && ((targetClass = obj.getClass()) != clazz)) {            // Must match a 2-list of { caller, targetClass }.            if (cache instanceof Class[]) {                Class<?>[] cache2 = (Class<?>[]) cache;                if (cache2[1] == targetClass &&                    cache2[0] == caller) {                    return;     // ACCESS IS OK                }                // (Test cache[1] first since range check for [1]                // subsumes range check for [0].)            }        } else if (cache == caller) {            // Non-protected case (or obj.class == this.clazz).            return;             // ACCESS IS OK        }        // If no return, fall through to the slow path.        slowCheckMemberAccess(caller, clazz, obj, modifiers, targetClass);    }

然后就是调用MethodAccessor的invoke方法了。

调用MethodAccessor的invoke方法。每个Method对象包含一个root对象，root对象里持有一个MethodAccessor对象。这个对象由ReflectionFactory方法生成,ReflectionFactory对象在Method类中是static final的由native方法实例化。代码片段如下;

//Method类中的代码片段，生成MethodAccessorprivate 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;    }// reflectionFactory在父类AccessibleObject中定义，代码片段如下：  static final ReflectionFactory reflectionFactory =        AccessController.doPrivileged(            new sun.reflect.ReflectionFactory.GetReflectionFactoryAction());

ReflectionFactory生成MethodAccessor：如果noInflation的属性为true则直接返回MethodAccessorGenerator创建的一个MethodAccessor，否则返回DelegatingMethodAccessorImpl，并将他与一个NativeMethodAccessorImpl互相引用。但DelegatingMethodAccessorImpl执行invoke方法的时候又委托给NativeMethodAccessorImpl了。代码片段如下：

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

MethodAccessor实现有两个版本，一个是Java实现的，另一个是native code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；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。

看下NativeMethodAccessorImpl 中的invoke方法：

代码片段如下：

package sun.reflect;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;class NativeMethodAccessorImpl extends MethodAccessorImpl{  private Method method;  private DelegatingMethodAccessorImpl parent;  private int numInvocations;  NativeMethodAccessorImpl(Method paramMethod)  {    this.method = paramMethod;  }  public Object invoke(Object paramObject, Object[] paramArrayOfObject)    throws IllegalArgumentException, InvocationTargetException  {    if (++this.numInvocations > ReflectionFactory.inflationThreshold()) {      MethodAccessorImpl localMethodAccessorImpl = (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(localMethodAccessorImpl);    }    return invoke0(this.method, paramObject, paramArrayOfObject);  }  void setParent(DelegatingMethodAccessorImpl paramDelegatingMethodAccessorImpl) {    this.parent = paramDelegatingMethodAccessorImpl;  }  private static native Object invoke0(Method paramMethod, Object paramObject, Object[] paramArrayOfObject);}

调用natiave方法invoke0执行方法调用.

注意这里有一个计数器numInvocations，每调用一次方法+1,当比 ReflectionFactory.inflationThreshold(15)大的时候,用MethodAccessorGenerator创建一个MethodAccessor,并把之前的DelegatingMethodAccessorImpl引用替换为现在新创建的。下一次DelegatingMethodAccessorImpl就不会再交给NativeMethodAccessorImpl执行了，而是交给新生成的java字节码的MethodAccessor

每次NativeMethodAccessorImpl.invoke()方法被调用时，都会增加一个调用次数计数器，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，是用C写的

为了验证这个结论，我故意写出一个非法参数，循环调用16次并catch下异常，结果如下：从结果中看出，前15次都是调用NativeMethodAccessorImpl，第16次开始就是调用DelegatingMethodAccessorImpl了。

java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentException: wrong number of argumentsat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)java.lang.IllegalArgumentExceptionat sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:606)at com.reflect.test.ReflectTest.main(ReflectTest.java:44)

下面看看java版的DelegatingMethodAccessorImpl的实现：

package sun.reflect;import java.lang.reflect.InvocationTargetException;class DelegatingMethodAccessorImpl extends MethodAccessorImpl{  private MethodAccessorImpl delegate;  DelegatingMethodAccessorImpl(MethodAccessorImpl paramMethodAccessorImpl)  {    setDelegate(paramMethodAccessorImpl);  }  public Object invoke(Object paramObject, Object[] paramArrayOfObject)    throws IllegalArgumentException, InvocationTargetException  {    return this.delegate.invoke(paramObject, paramArrayOfObject);  }  void setDelegate(MethodAccessorImpl paramMethodAccessorImpl) {    this.delegate = paramMethodAccessorImpl;  }package sun.reflect;public class GeneratedMethodAccessor1 extends MethodAccessorImpl {        public GeneratedMethodAccessor1() {        super();    }        public Object invoke(Object obj, Object[] args)           throws IllegalArgumentException, InvocationTargetException {        // prepare the target and parameters        if (obj == null) throw new NullPointerException();        try {            A target = (A) obj;            if (args.length != 1) throw new IllegalArgumentException();            String arg0 = (String) args[0];        } catch (ClassCastException e) {            throw new IllegalArgumentException(e.toString());        } catch (NullPointerException e) {            throw new IllegalArgumentException(e.toString());        }        // make the invocation        try {            target.foo(arg0);        } catch (Throwable t) {            throw new InvocationTargetException(t);        }    }}

if (args.length != 1) throw new IllegalArgumentException();这一句就能解释我之前的疑问了，这块会判断参数数组的长度，如果长度不等于1，就会抛出非法参数的异常。

而且MethodAccessor会做强制类型转换再进行方法调用，但父类强制转化成子类的的时候就会报错类型不匹配错误了，所以如果变量的引用声明是父但实际指向的对象是子，那么这种调用也是可以的。