java反射机制之Method invoke执行调用方法例子

àì夳堔傛蜴生んèń 2022-07-16 11:22 220阅读 0赞

原文：[java反射机制之Method invoke执行调用方法例子][java_Method invoke]

源代码下载地址： [http://www.zuidaima.com/share/1843292395834368.htm][java_Method invoke]

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。

[java_Method invoke]: http://www.zuidaima.com/share/1843292395834368.htm