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
1 | package com.reflect.test; |
3 | public class BaseObject { |
5 | public void getObjectName(){ |
6 | System.out.println( "BaseObject" ); |
01 | package com.reflect.test; |
03 | public class SubObject extends BaseObject{ |
05 | public void getObjectName() { |
06 | System.out.println( "SubObject" ); |
08 | public void getParamsLength(String...params){ |
09 | System.out.println( "param's length is:" +params.length); |
11 | public void getParamsLength(String param1,String param2){ |
12 | 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
01 | package com.reflect.test; |
03 | import java.lang.reflect.Method; |
05 | public class ReflectTest { |
07 | private static final String BASE_OBJECT_PATH = "com.reflect.test.BaseObject" ; |
08 | private static final String SUB_OBJECT_PATH = "com.reflect.test.SubObject" ; |
10 | public static void main(String[] args) throws Exception{ |
12 | Class<?> bClazz = Class.forName(BASE_OBJECT_PATH); |
13 | Class<?> sClazz = Class.forName(SUB_OBJECT_PATH); |
15 | Object bObj = bClazz.newInstance(); |
16 | Object sObj = sClazz.newInstance(); |
20 | Method bMethod = bClazz.getDeclaredMethod( "getObjectName" ); |
24 | Method sMethod = sClazz.getDeclaredMethod( "getObjectName" ); |
30 | Method changeMethod = sClazz.getDeclaredMethod( "getParamsLength" , String[]. class ); |
32 | String[] strParams = { "a" , "b" , "c" }; |
33 | Object[] cParams = {strParams}; |
34 | changeMethod.invoke(sObj, cParams); |
37 | Method unChangeMethod1 = sClazz.getDeclaredMethod( "getParamsLength" , String. class ,String. class ); |
38 | unChangeMethod1.invoke(sObj, "Hello" , "Java" ); |
40 | Class<?>[] clazzs = {String. class ,String. class }; |
41 | Method unChangeMethod2 = sClazz.getDeclaredMethod( "getParamsLength" , clazzs); |
42 | unChangeMethod2.invoke(sObj, "Hello" , "Java" ); |
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。
02 | public Object invoke(Object obj, Object... args) |
03 | throws IllegalAccessException, IllegalArgumentException, |
04 | InvocationTargetException |
07 | if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { |
12 | Class<?> caller = getCallerClass(); |
13 | checkAccess(caller, clazz, obj, modifiers); |
16 | MethodAccessor ma = methodAccessor; |
18 | ma = acquireMethodAccessor(); |
20 | return ma.invoke(obj, args); |
24 | volatile Object securityCheckCache; |
26 | void checkAccess(Class<?> caller, Class<?> clazz, Object obj, int modifiers) |
27 | throws IllegalAccessException |
29 | if (caller == clazz) { |
32 | Object cache = securityCheckCache; |
33 | Class<?> targetClass = clazz; |
35 | && Modifier.isProtected(modifiers) |
36 | && ((targetClass = obj.getClass()) != clazz)) { |
38 | if (cache instanceof Class[]) { |
39 | Class<?>[] cache2 = (Class<?>[]) cache; |
40 | if (cache2[ 1 ] == targetClass && |
41 | cache2[ 0 ] == caller) { |
47 | } else if (cache == caller) { |
53 | 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;
02 | private volatile MethodAccessor methodAccessor; |
04 | private MethodAccessor acquireMethodAccessor() { |
07 | MethodAccessor tmp = null ; |
08 | if (root != null ) tmp = root.getMethodAccessor(); |
13 | tmp = reflectionFactory.newMethodAccessor( this ); |
14 | setMethodAccessor(tmp); |
21 | static final ReflectionFactory reflectionFactory = |
22 | AccessController.doPrivileged( |
23 | 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:
01 | public MethodAccessor newMethodAccessor(Method paramMethod) { |
05 | return new MethodAccessorGenerator().generateMethod(paramMethod.getDeclaringClass(), paramMethod.getName(), paramMethod.getParameterTypes(), paramMethod.getReturnType(), paramMethod.getExceptionTypes(), paramMethod.getModifiers()); |
08 | NativeMethodAccessorImpl localNativeMethodAccessorImpl = new NativeMethodAccessorImpl(paramMethod); |
10 | DelegatingMethodAccessorImpl localDelegatingMethodAccessorImpl = new DelegatingMethodAccessorImpl(localNativeMethodAccessorImpl); |
12 | localNativeMethodAccessorImpl.setParent(localDelegatingMethodAccessorImpl); |
13 | 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:
03 | import java.lang.reflect.InvocationTargetException; |
04 | import java.lang.reflect.Method; |
06 | class NativeMethodAccessorImpl extends MethodAccessorImpl |
08 | private Method method; |
09 | private DelegatingMethodAccessorImpl parent; |
10 | private int numInvocations; |
12 | NativeMethodAccessorImpl(Method paramMethod) |
14 | this .method = paramMethod; |
17 | public Object invoke(Object paramObject, Object[] paramArrayOfObject) |
18 | throws IllegalArgumentException, InvocationTargetException |
20 | if (++ this .numInvocations > ReflectionFactory.inflationThreshold()) { |
21 | MethodAccessorImpl localMethodAccessorImpl = (MethodAccessorImpl) new MethodAccessorGenerator().generateMethod( this .method.getDeclaringClass(), this .method.getName(), this .method.getParameterTypes(), this .method.getReturnType(), this .method.getExceptionTypes(), this .method.getModifiers()); |
23 | this .parent.setDelegate(localMethodAccessorImpl); |
26 | return invoke0( this .method, paramObject, paramArrayOfObject); |
29 | void setParent(DelegatingMethodAccessorImpl paramDelegatingMethodAccessorImpl) { |
30 | this .parent = paramDelegatingMethodAccessorImpl; |
33 | 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。
01 | java.lang.IllegalArgumentException: wrong number of arguments |
02 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
03 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
04 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
05 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
06 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
07 | java.lang.IllegalArgumentException: wrong number of arguments |
08 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
09 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
10 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
11 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
12 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
13 | java.lang.IllegalArgumentException: wrong number of arguments |
14 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
15 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
16 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
17 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
18 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
19 | java.lang.IllegalArgumentException: wrong number of arguments |
20 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
21 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
22 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
23 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
24 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
25 | java.lang.IllegalArgumentException: wrong number of arguments |
26 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
27 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
28 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
29 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
30 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
31 | java.lang.IllegalArgumentException: wrong number of arguments |
32 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
33 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
34 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
35 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
36 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
37 | java.lang.IllegalArgumentException: wrong number of arguments |
38 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
39 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
40 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
41 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
42 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
43 | java.lang.IllegalArgumentException: wrong number of arguments |
44 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
45 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
46 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
47 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
48 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
49 | java.lang.IllegalArgumentException: wrong number of arguments |
50 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
51 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
52 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
53 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
54 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
55 | java.lang.IllegalArgumentException: wrong number of arguments |
56 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
57 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
58 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
59 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
60 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
61 | java.lang.IllegalArgumentException: wrong number of arguments |
62 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
63 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
64 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
65 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
66 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
67 | java.lang.IllegalArgumentException: wrong number of arguments |
68 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
69 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
70 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
71 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
72 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
73 | java.lang.IllegalArgumentException: wrong number of arguments |
74 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
75 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
76 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
77 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
78 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
79 | java.lang.IllegalArgumentException: wrong number of arguments |
80 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
81 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
82 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
83 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
84 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
85 | java.lang.IllegalArgumentException: wrong number of arguments |
86 | at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) |
87 | at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java: 57 ) |
88 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
89 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
90 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
91 | java.lang.IllegalArgumentException |
92 | at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source) |
93 | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java: 43 ) |
94 | at java.lang.reflect.Method.invoke(Method.java: 606 ) |
95 | at com.reflect.test.ReflectTest.main(ReflectTest.java: 44 ) |
下面看看java版的DelegatingMethodAccessorImpl的实现:
03 | import java.lang.reflect.InvocationTargetException; |
05 | class DelegatingMethodAccessorImpl extends MethodAccessorImpl |
07 | private MethodAccessorImpl delegate; |
09 | DelegatingMethodAccessorImpl(MethodAccessorImpl paramMethodAccessorImpl) |
11 | setDelegate(paramMethodAccessorImpl); |
14 | public Object invoke(Object paramObject, Object[] paramArrayOfObject) |
15 | throws IllegalArgumentException, InvocationTargetException |
17 | return this .delegate.invoke(paramObject, paramArrayOfObject); |
20 | void setDelegate(MethodAccessorImpl paramMethodAccessorImpl) { |
21 | this .delegate = paramMethodAccessorImpl; |
27 | public class GeneratedMethodAccessor1 extends MethodAccessorImpl { |
28 | public GeneratedMethodAccessor1() { |
32 | public Object invoke(Object obj, Object[] args) |
33 | throws IllegalArgumentException, InvocationTargetException { |
35 | if (obj == null ) throw new NullPointerException(); |
38 | if (args.length != 1 ) throw new IllegalArgumentException(); |
39 | String arg0 = (String) args[ 0 ]; |
40 | } catch (ClassCastException e) { |
41 | throw new IllegalArgumentException(e.toString()); |
42 | } catch (NullPointerException e) { |
43 | throw new IllegalArgumentException(e.toString()); |
48 | } catch (Throwable t) { |
49 | 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。
由最代码官方编辑于2016-10-17 9:38:13