Five years ago, I published an article in Hungarian about how to alter the IntegerCahe in the JDK. Doing that is essentially hacking the Java run-time, and there is no practical advantage unless, while you develop the hacking code, you get at better understanding how reflection works and how the Integer class is implemented.

The Integer class has a private nested class named IntegerCache that contains Integer objects for the int values -127 to 128. When the code has to box an int to Integer and the value is within this range, then the run-time uses the cache instead of creating a new Integer object. This is done for speed optimization reasons, bearing in mind that the int values in programs are, many times, in this range (think about array indexing).

The side effect of this is that many times, using the identity operator to compare two Integer objects may work so long as long the value is in the range. This is typically during unit testing. In operational mode, when some of the values get bigger than 128, the code fails.

Hacking the IntegerCache using reflection may also lead to mysterious side effects, and note that this is something that has its effect on the whole JVM. If a servlet redefines the small Integer cached values, then all other servlets running in the same Tomcat under the same JVM will suffer.

There are other articles about it on the net from Lukas Eder and on Sitepoint.

Now that I've started to play around with the Java 9 early access version, it came to my mind that I can do the same hacking with the new version of Java. Before starting that, let’s refresh what we did with Java 8.

Lukas, in his article, displays sample code that I'm copying here:

import java.lang.reflect.Field;
import java.util.Random;

public class Entropy {
    public static void main(String[] args)
    throws Exception {

        // Extract the IntegerCache through reflection
        Class << ? > clazz = Class.forName(
            "java.lang.Integer$IntegerCache");
        Field field = clazz.getDeclaredField("cache");
        field.setAccessible(true);
        Integer[] cache = (Integer[]) field.get(clazz);

        // Rewrite the Integer cache
        for (int i = 0; i < cache.length; i++) {
            cache[i] = new Integer(
                new Random().nextInt(cache.length));
        }

        // Prove randomness
        for (int i = 0; i < 10; i++) {
            System.out.println((Integer) i);
        }
    }
}

The code gets access to the IntegerCache via reflection and then fills the cache with random values. Naughty.

We can try to execute the same code in Java 9. Do not expect much fun though. Java 9 is more serious when somebody tries to violate it.

Exception in thread "main" java.lang.reflect.InaccessibleObjectException:
  Unable to make field static final java.lang.Integer[]
  java.lang.Integer$IntegerCache.cache
  accessible: module java.base does not "opens java.lang" to unnamed module @1bc6a36e

We get an exception that did not exist in Java 8. It says that the object is not accessible because the module java.base, which is the part of the run-time of the JDK that is automatically imported by each Java program, does not ‘opens’ (sic) the module to the unnamed module. It is thrown from the line where we try to set the field accessible.

The object we could easily access in Java 8 is not accessible anymore because the module system protects it. Code can only access fields, methods, and other things using reflection, only if the class is in the same module, or if the module opens the package for reflective access for the world or for the module that needs the access. This is done in the module-info.java module definition file:

module myModule {
    exports com.javax0.module.demo;
    opens com.javax0.module.demo;
}

The module java.base does not open itself for reflective access for us and especially not for the unnamed module, which is the code that we run. If we create a module for our code and name it, then the error message will contain the name of that module.

Can we open the module programmatic? There is an addOpens method in the java.lang.reflect.Module module.

Will it work?

Bad news for the hackers: It will not. It can only open a package in a module to another module if that package is already opened for the module calling this method. That way, modules can pass on to other modules the right that they already have to access some packages in a reflective way but can not open things that are not open.

But at the same time, this is good news. Java 9 is not so easily hackable, like Java 8 was. At least this little hole is closed. It seems that Java has started to be professional grade and not a toy. Soon the time will come when we can migrate serious programs from RPG and COBOL to Java. (Sorry for the joke.)