Why Java 9 does not simply turn all JARs on the class path into automatic modules?

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In order to understand the categories we have:

  • platform explicit modules
  • application explicit modules
  • open modules
  • automatic modules
  • unnamed module

All classes and jars within the classpath will be part of the unnamed module. But why is that what we need? Where is the advantage over automatic modules? I could "require" those damn legacy jars to make them to an automatic module. Do I not have included everything with it?

java-9 java-platform-module-system java
2 Answers

Apart from the items listed in the accepted answer, there is one more difference: unnamed modules can access all packages of modules that come with Java, even is they are not exported. As long the class is public, access will work - the same as before Java 9. But as soon as a jar is run from module path, it will be able to access only exported packages.

For example if some .jar has this code:

com.sun.jmx.remote.internal.ArrayQueue c = new com.sun.jmx.remote.internal.ArrayQueue(10);

it will run normally without any warnings when placed on class path, but when run from module path (as automatic module) it will fail at runtime:

Exception in thread "main" java.lang.IllegalAccessError: class test1.C 
(in module test1) cannot access class com.sun.jmx.remote.internal.ArrayQueue 
(in module java.management) because module java.management does not export 
com.sun.jmx.remote.internal to module test1

Note that this is different from the well known illegal reflective access warning, which is about using reflection to access private fields or methods. Here we are statically (non-reflectively) accessing public class (but from non-exported package).

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