[英]How do I access private methods and private data members via reflection?
我知道我們可以通過反射來訪問私有構造函數@Sanjay T. Sharma在他的問題回答中提到: “instanceof Void”總是返回false嗎?
你可以使用反射訪問私有 - 方法,構造函數,數據成員,一切。
1) 如何訪問私有方法和私有數據成員?
你可以在setAccessible(true)
方法的幫助下完成它:
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) 是否可以通過反射訪問局部變量?
不能。局部變量不能在創建它們的塊之外訪問(有人可以說你可以將這樣的變量分配給像field = localVariable;
這樣的字段field = localVariable;
然后通過反射訪問這樣的字段,但這樣我們就會訪問值 ,而不是變量 )。
3) 有沒有辦法阻止任何人訪問私有構造函數,方法和數據成員?
我認為對於constructors
或methods
您可以使用stacktrace來檢查它是否被Reflection
調用。
對於字段,我找不到阻止通過反射訪問它們的解決方案。
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
主要方法的輸出 :
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
要訪問 私有字段,您需要調用Class.getDeclaredField(String name)
或enter code here
方法。 檢查這個簡單的代碼:
public class PrivateObject {
private String privateString = null;
public PrivateObject(String privateString) {
this.privateString = privateString;
}
}
PrivateObject privateObject = new PrivateObject("The Private Value");
Field privateStringField = PrivateObject.class.
getDeclaredField("privateString");
privateStringField.setAccessible(true);
String fieldValue = (String) privateStringField.get(privateObject);
System.out.println("fieldValue = " + fieldValue
要訪問私有方法,您需要調用Class.getDeclaredMethod(String name,Class [] parameterTypes)或Class.getDeclaredMethods()方法。
檢查這個簡單的代碼:
public class PrivateObject {
private String privateString = null;
public PrivateObject(String privateString) {
this.privateString = privateString;
}
private String getPrivateString(){
return this.privateString;
}
}
PrivateObject privateObject = new PrivateObject("The Private Value");
Method privateStringMethod = PrivateObject.class.
getDeclaredMethod("getPrivateString", null);
privateStringMethod.setAccessible(true);
String returnValue = (String)
privateStringMethod.invoke(privateObject, null);
System.out.println("returnValue = " + returnValue);
請閱讀http://tutorials.jenkov.com/java-reflection/private-fields-and-methods.html上的詳細信息
Area s=(Area)c.newInstance();
s.setRadius(10);
System.out.println("Area: "+s.calculateArea(4));
Method m[]=c.getDeclaredMethods();
Constructor c1[]=c.getConstructors();
for(int i=0;i<m.length;i++)
System.out.println(""+m[i]);
for(int i=0;i<c1.length;i++)
System.out.println(""+c1[i]);
示例如下:
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
class Test
{
private int a = 5; // Private data member
private void call(int n) // Private method
{
System.out.println("in call() n: " + n);
}
}
public class Sample
{
public static void main(String args[]) throws ClassNotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchFieldException
{
Class c = Class.forName("Test");
Object obj = c.newInstance();
//---- Accessing a private method
Method m=c.getDeclaredMethod("call",new Class[]{int.class});
m.setAccessible(true);
m.invoke(obj,7);
//---- Accessing a private data member
Field d = c.getDeclaredField("a");
d.setAccessible(true);
System.out.println(d.getInt(obj));
}
}
回答你的第三個問題:
回答:
是的,您可以限制訪問權限(當有人試圖訪問您的私有構造函數/方法/數據時,您可以拋出異常)
請參考以下示例:
******JavaSingleton Class******
package server;
public class JavaSingleton {
private static final JavaSingleton INSTANCE = new JavaSingleton();
private JavaSingleton() {
if (INSTANCE != null) {
throw new IllegalStateException("Inside JavaSingleton(): JavaSingleton " +
"instance already created.");
}
System.out.println("Inside JavaSingleton(): Singleton instance is being created.");
}
public static final JavaSingleton getInstance() {
return INSTANCE;
}
}
***Listing 2: JavaSingleton client***
import server.JavaSingleton;
import java.lang.reflect.*;
public class TestSingleton {
public static void main(String[] args) throws ReflectiveOperationException {
System.out.println("Inside main(): Getting the singleton instance using getInstance()...");
JavaSingleton s = JavaSingleton.getInstance();
System.out.println("Inside main(): Trying to use reflection to get another instance...");
Class<JavaSingleton> clazz = JavaSingleton.class;
Constructor<JavaSingleton> cons = clazz.getDeclaredConstructor();
cons.setAccessible(true);
JavaSingleton s2 = cons.newInstance();
}
}
Output:
C:\singleton>java TestSingleton
Inside main(): Getting the singleton instance using getInstance()...
Inside JavaSingleton(): Singleton instance is being created.
Inside main(): Trying to use reflection to get another instance...
Exception in thread "main" java.lang.reflect.InvocationTargetException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at TestSingleton.main(TestSingleton.java:13)
Caused by: java.lang.IllegalStateException: Inside JavaSingleton(): JavaSingleton instance already created.
at server.JavaSingleton.<init>(JavaSingleton.java:7)
... 5 more
此示例適用於單例類(檢查構造函數),但您仍可以為要阻止從其他類訪問的私有方法實現此邏輯。
在這種情況下,您還將聲明一個靜態實例並在私有方法中檢查它的值,並在出現任何不需要的值時拋出錯誤。
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