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copied!<p>1) <strong><em>How can I access the private methods and the private data members?</em></strong></p> <p>You can do it with a little help of the <code>setAccessible(true)</code> method:</p> <pre><code>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)); } } </code></pre> <p>2) <strong><em>Is it possible to access a local variable via reflection?</em></strong></p> <p>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 <code>field = localVariable;</code> and later access such a field via reflection, but this way we will be accessing the <em>value</em>, not the <em>variable</em>).</p> <p>3) <strong><em>Is there any way to prevent anyone from accessing private constructors, methods, and data members?</em></strong></p> <p>I think for <code>constructors</code> or <code>methods</code> you could use stacktrace to check if it was invoked by <code>Reflection</code>. <br/>For fields I can't find a solution to prevent accessing them via reflection.</p> <h2>[WARNING: This is not approved by anyone. I just wrote it inspired by your question.]</h2> <pre><code>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); } } </code></pre> <p><strong>Output from <code>Test</code> main method</strong>:</p> <pre><code>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 </code></pre>

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