kaka的gravatar头像
kaka 2014-05-29 13:16:54

java反射机制之Method invoke执行调用方法例子

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{
	@Override
	public 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 class
		sMethod.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MethodAccessor
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;
    }

// 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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 arguments
	at 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
	at 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。


最代码官方编辑于2016-10-17 9:38:13


打赏

最代码最近下载分享源代码列表最近下载
最代码最近浏览分享源代码列表最近浏览
浙江螃蟹  LV7 2021年3月21日
starhbx 2021年2月4日
暂无贡献等级
LiXin1994  LV1 2020年12月9日
小小123123  LV1 2020年11月17日
寳寶88  LV1 2020年8月9日
zist_xym  LV1 2020年3月23日
xp95323  LV14 2019年12月3日
HHLee1995  LV1 2019年11月21日
luohaipeng  LV23 2019年11月20日
胖胖来了  LV4 2019年11月7日
顶部 客服 微信二维码 底部
>扫描二维码关注最代码为好友扫描二维码关注最代码为好友