如何通过反射访问私有方法和私有数据成员? [英] How do I access private methods and private data members via reflection?
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问题描述
我知道我们可以通过反射访问私有构造函数，如 @Sanjay T. Sharma 所述.我的问题的答案:"instanceof Void"是否总是返回false?
1) 如何访问私有方法和私有数据成员?
您可以在setAccessible(true)方法的一点帮助下完成此操作:
class Dummy{
private void foo(){
System.out.println("hello foo()");
}
private int i = 10;
}
class Test{
public static void main(String[] args) throws Exception {
Dummy d = new Dummy();
/*--- [INVOKING PRIVATE METHOD] ---*/
Method m = Dummy.class.getDeclaredMethod("foo");
//m.invoke(d); // Exception java.lang.IllegalAccessException
m.setAccessible(true);//Abracadabra
m.invoke(d); // Now it's OK
/*--- [GETING VALUE FROM PRIVATE FIELD] ---*/
Field f = Dummy.class.getDeclaredField("i");
//System.out.println(f.get(d)); // Not accessible now
f.setAccessible(true); // Abracadabra
System.out.println(f.get(d)); // Now it's OK
/*--- [SETTING VALUE OF PRIVATE FIELD] ---*/
Field f2 = Dummy.class.getDeclaredField("i");
//f2.set(d,20); // Not accessible now
f2.setAccessible(true); // Abracadabra
f2.set(d, 20); // Now it's OK
System.out.println(f2.get(d));
}
}
2) 是否可以通过反射访问局部变量?
不.不能在创建它们的块之外访问局部变量(有人可以说您可以将这样的变量分配给field = localVariable;之类的字段，然后再通过反射来访问该字段，但是这种方式我们将访问 value ，而不是 variable ).
3) 是否可以阻止任何人访问私有构造函数，方法和数据成员?
我认为对于constructors或methods，您可以使用stacktrace来检查它是否由Reflection调用.
对于字段，我找不到阻止通过反射访问它们的解决方案.
[警告:未经任何人的认可.我只是根据您的问题写的.]
class Dummy {
private void safeMethod() {
StackTraceElement[] st = new Exception().getStackTrace();
// If a method was invoked by reflection, the stack trace would be similar
// to something like this:
/*
java.lang.Exception
at package1.b.Dummy.safeMethod(SomeClass.java:38)
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:601)
at package1.b.Test.main(SomeClass.java:65)
*/
//5th line marked by "->" is interesting one so I will try to use that info
if (st.length > 5 &&
st[4].getClassName().equals("java.lang.reflect.Method"))
throw new RuntimeException("safeMethod() is accessible only by Dummy object");
// Now normal code of method
System.out.println("code of safe method");
}
// I will check if it is possible to normally use that method inside this class
public void trySafeMethod(){
safeMethod();
}
Dummy() {
safeMethod();
}
}
class Dummy1 extends Dummy {}
class Test {
public static void main(String[] args) throws Exception {
Dummy1 d1 = new Dummy1(); // safeMethod can be invoked inside a superclass constructor
d1.trySafeMethod(); // safeMethod can be invoked inside other Dummy class methods
System.out.println("-------------------");
// Let's check if it is possible to invoke it via reflection
Method m2 = Dummy.class.getDeclaredMethod("safeMethod");
// m.invoke(d);//exception java.lang.IllegalAccessException
m2.setAccessible(true);
m2.invoke(d1);
}
}
Test主要方法的输出:
code of safe method
code of safe method
-------------------
Exception in thread "main" java.lang.reflect.InvocationTargetException
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:601)
at package1.b.Test.main(MyClass2.java:87)
Caused by: java.lang.RuntimeException: method safeMethod() is accessible only by Dummy object
at package1.b.Dummy.safeMethod(MyClass2.java:54)
... 5 more
I know that we can access private constructor via reflection as @Sanjay T. Sharma mentioned in his answer of my question: Does "instanceof Void" always return false?
you can access private everything with reflection - methods, constructors, data members, everything.
- How can I access the private methods and the private data members?
- Is it possible to access local variable via reflection?
- Is there a way to prevent anyone from accessing private constructors, methods, and data members?
解决方案
1) How can I access the private methods and the private data members?
You can do it with a little help of the setAccessible(true) method:
class Dummy{
private void foo(){
System.out.println("hello foo()");
}
private int i = 10;
}
class Test{
public static void main(String[] args) throws Exception {
Dummy d = new Dummy();
/*--- [INVOKING PRIVATE METHOD] ---*/
Method m = Dummy.class.getDeclaredMethod("foo");
//m.invoke(d); // Exception java.lang.IllegalAccessException
m.setAccessible(true);//Abracadabra
m.invoke(d); // Now it's OK
/*--- [GETING VALUE FROM PRIVATE FIELD] ---*/
Field f = Dummy.class.getDeclaredField("i");
//System.out.println(f.get(d)); // Not accessible now
f.setAccessible(true); // Abracadabra
System.out.println(f.get(d)); // Now it's OK
/*--- [SETTING VALUE OF PRIVATE FIELD] ---*/
Field f2 = Dummy.class.getDeclaredField("i");
//f2.set(d,20); // Not accessible now
f2.setAccessible(true); // Abracadabra
f2.set(d, 20); // Now it's OK
System.out.println(f2.get(d));
}
}
2) Is it possible to access a local variable via reflection?
No. Local variables cannot be accessed outside of a block in which they were created (someone could say that you can assign such a variable to a field like field = localVariable; and later access such a field via reflection, but this way we will be accessing the value, not the variable).
3) Is there any way to prevent anyone from accessing private constructors, methods, and data members?
I think for constructors or methods you could use stacktrace to check if it was invoked by Reflection.
For fields I can't find a solution to prevent accessing them via reflection.
[WARNING: This is not approved by anyone. I just wrote it inspired by your question.]
class Dummy {
private void safeMethod() {
StackTraceElement[] st = new Exception().getStackTrace();
// If a method was invoked by reflection, the stack trace would be similar
// to something like this:
/*
java.lang.Exception
at package1.b.Dummy.safeMethod(SomeClass.java:38)
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:601)
at package1.b.Test.main(SomeClass.java:65)
*/
//5th line marked by "->" is interesting one so I will try to use that info
if (st.length > 5 &&
st[4].getClassName().equals("java.lang.reflect.Method"))
throw new RuntimeException("safeMethod() is accessible only by Dummy object");
// Now normal code of method
System.out.println("code of safe method");
}
// I will check if it is possible to normally use that method inside this class
public void trySafeMethod(){
safeMethod();
}
Dummy() {
safeMethod();
}
}
class Dummy1 extends Dummy {}
class Test {
public static void main(String[] args) throws Exception {
Dummy1 d1 = new Dummy1(); // safeMethod can be invoked inside a superclass constructor
d1.trySafeMethod(); // safeMethod can be invoked inside other Dummy class methods
System.out.println("-------------------");
// Let's check if it is possible to invoke it via reflection
Method m2 = Dummy.class.getDeclaredMethod("safeMethod");
// m.invoke(d);//exception java.lang.IllegalAccessException
m2.setAccessible(true);
m2.invoke(d1);
}
}
Output from Test main method:
code of safe method
code of safe method
-------------------
Exception in thread "main" java.lang.reflect.InvocationTargetException
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:601)
at package1.b.Test.main(MyClass2.java:87)
Caused by: java.lang.RuntimeException: method safeMethod() is accessible only by Dummy object
at package1.b.Dummy.safeMethod(MyClass2.java:54)
... 5 more
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