Friday, May 28, 2004

Instantiate a Java class that has private constructor

Java reflection is like double edged sword. If you don't use it properly, you would end up in a code mess. Remember the GOTO phenomenon during the BASIC era? Here is one example that I found today. Have you ever have a need to instantiate a class that has private constructor? There are two things you can do :

1. Sit back and think; if you ever have a need to instantiate that class, maybe that class should have a public constructor in the first place. Or maybe make it as interface or abstract class.

2. Or, you can use the mighty Java Reflection. Here is an example :

public class Secret {
   private Secret() {
      // some secret

public class Hacker {
   public static void main(String args[]) throws Exception {
      Constructor[] cons = Secret.class.getDeclaredConstructors();
      // Change the accessible property of the constructor.
      Secret secret = (Secret)cons[0].newInstance(null);
      // You are in !

One caveat is it will not work if you have SecurityManager installed.


At 6:48 PM, Blogger Prithvi said...


Could you throw light on how to disable the security manager?


At 10:25 PM, Anonymous Anonymous said...

One problem you have is that the default constructor is not guaranteed to be at position 0 in the array.

You can easily set all of the constructors accessible with the static method:

AccessibleObject.setAccessible(cons, true);

At 10:27 PM, Anonymous Anonymous said...

I do realize that you only have one constructor in your example, but it might not be clear that if you were to add another constructor, your solution would not always work.

At 5:06 AM, Anonymous Steff said...

You could easily search for the default constructor by doing the following:

// Search for the default constructor
for (int i = 0; i < constructors.length; i++) {
if (constructors[i].getParameterTypes().length == 0) {
defaultConstructor = constructors[i];

At 2:29 PM, Anonymous Ivan said...

This works for me only when I use getDeclaredConstructor().

No wonder, the javadoc states, that getConstructor() only returns public constructors.

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