Dangerous drop-in replacement Jars

Monday 04 February 2013

Have you ever put an updated jar into your classpath, eg. a new version of a library for your webapp? In this post I’d like to show you something you should be aware of, especially if you get strange behavior after a simple jar update.

The riddle

Create an interface:

public interface MyInterface {
String GREETING = "Hello World";
}

Now we compile the interface and pack it into a jar.

> javac MyInterface.java
> jar -cf lib.jar MyInterface.class

Next is a simple demo application

public class App {
public static void main(String[] args) {
System.out.println("Greeting: " + MyInterface.GREETING);
}
}

The app gets compiled using the library-jar we created before and pack it.

> javac -cp lib.jar App.java
> jar -cf app.jar App.class

Now that the demo app is ready we start it with the command:

> java -cp app.jar:lib.jar App

What a suprise, the output is as expected:

Greeting: Hello World

Now the fun starts. Edit the MyInterface, change the value of GREETING.

public interface MyInterface {
    String GREETING = "Good Bye";
}

Recompile and repack our interface

> javac MyInterface.java
> jar -cf drop-in.jar MyInterface.class

And now, here comes the million dollar question! What will we get if we start our app with the new drop-in-jar instead of the old lib.jar in the classpath?

> java -cp app.jar:drop-in.jar App

Suggestions?

What do you guess? Greeting: Good Bye? Wrong! We still get Greeting: Hello World even though our interface in the classpath says good bye!

What did just happen?

For optimization reasons the java compiler writes our constant value directly into the App bytecode. Java doesn’t read it from the interface at runtime! Therefore, the value from the drop-in jar is never read at runtime.

Conclusion

When using drop-in replacement jars keep in mind that changed constant values of the library will only make it into the runtime if you recompile your code!

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