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README.md

Class Loading

There are several changes with regards to class loading, but so far we've only documented one.

Restricted Bootstrap Class Loader

A common requirement in server and testing frameworks is the isolation of external code from the framework. Before Java 9, this could be done by loading the external code via a URLClassLoader with a null parent class loader, which limits it to just the classes of the runtime Java platform and the class path passed to the URLClassLoader. A null parent class loader signifies the bootstrap class loader.

The set of classes that the bootstrap class loader is able to load has changed between Java 8 and Java 9:

  • In Java 8, every platform class other than the JavaFX classes is visible to the bootstrap class loader.
  • In Java 9, the bootstrap class loader is more restricted.

The list of modules that go into the bootstrap class loader can be found in JEP 261:

  • java.base
  • java.datatransfer
  • java.desktop
  • java.instrument
  • java.logging
  • java.management
  • java.management.rmi
  • java.naming
  • java.prefs
  • java.rmi
  • java.security.sasl
  • java.xml
  • jdk.httpserver
  • jdk.internal.vm.ci
  • jdk.management
  • jdk.management.agent
  • jdk.naming.rmi
  • jdk.net
  • jdk.sctp
  • jdk.unsupported

As a demonstration, BootstrapLoaderTest attempts to load classes from several of the non-core java/javax packages to validate what packages are visible to the bootstrap class loader. This shows the following packages that were visible under Java 8 to no longer be visible under Java 9:

  • java.sql.*
  • javax.activation.*
  • javax.annotation.*
  • javax.jws.*
  • javax.lang.model.*
  • javax.rmi.*
  • javax.script.*
  • javax.smartcardio.*
  • javax.sql.*
  • javax.tools.*
  • javax.transaction.xa.*
  • javax.xml.bind.*
  • javax.xml.crypto.*
  • javax.xml.soap.*
  • javax.xml.ws.*

NullParentClassLoaderIT shows the consequences by trying to load a class from the project JAR that uses java.sql.Date, which is among the invisible packages.

To observe that, you need to successfully build the JAR, either by building on Java 8 or running mvn package -DskipTests on Java 9 (otherwise BootstrapLoaderTest fails your build). Either way, you can then run the test with mvn failsafe:integration-test.

Solution

On Java 9, the new ClassLoader::getPlatformClassLoader, method must be used to obtain a parent that has visibility to all of the platform classes. It can then be passed to the URLClassLoader:

URL path[] = { ... };
ClassLoader parent = ClassLoader.getPlatformClassLoader();
URLClassLoader loader = new URLClassLoader(path, parent);

(Last checked: 8u152 and 9.0.1; contributed by Scott Stark)