我知道我们可以通过反射访问私有构造函数,正如@Sanjay T. Sharma在他对我的问题的回答中提到的那样:“instanceof Void”是否总是返回 false?
你可以通过反射访问私有的一切——方法、构造函数、数据成员,一切。
- 如何访问私有方法和私有数据成员?
- 是否可以通过反射访问局部变量?
- 有没有办法阻止任何人访问私有构造函数、方法和数据成员?
我知道我们可以通过反射访问私有构造函数,正如@Sanjay T. Sharma在他对我的问题的回答中提到的那样:“instanceof Void”是否总是返回 false?
你可以通过反射访问私有的一切——方法、构造函数、数据成员,一切。
1)如何访问私有方法和私有数据成员?
您可以借助以下方法来做到这一点setAccessible(true)
:
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。局部变量不能在创建它们的块之外访问(有人可能会说,您可以将这样的变量分配给类似的字段field = localVariable;
,然后通过反射访问这样的字段,但是这样我们将访问值,不是变量)。
3)有没有办法阻止任何人访问私有构造函数、方法和数据成员?
我认为 forconstructors
或者methods
您可以使用 stacktrace 来检查它是否被Reflection
.
对于字段,我找不到阻止通过反射访问它们的解决方案。
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
要访问 私有字段,您需要调用
Class.getDeclaredField(String name)
orenter code here
方法。
检查这个简单的代码:
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
要访问私有方法,您需要调用 Class.getDeclaredMethod(String name, Class[] parameterTypes) 或 Class.getDeclaredMethods() 方法。
检查这个简单的代码:
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);
在http://tutorials.jenkov.com/java-reflection/private-fields-and-methods.html阅读详细信息
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));
}
}
回答你的第三个问题:
回答:
是的,您可以限制访问(当有人试图访问您的私有构造函数/方法/数据时,您可以抛出异常)
请参考以下示例:
******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
此示例适用于单例类(检查构造函数),但您仍然可以为要阻止其他类访问的私有方法实现此逻辑。
在这种情况下,您还将声明一个静态实例并在私有方法中检查它的值,并在出现任何不需要的值时抛出错误。