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EclipseLink/Development/Dynamic/def1
Dynamic Persistence
Dynamic Persistence is defined as the ability to create a persistent entity class and use it within an application without a-priori the Java class existing (no .class file on the classpath or in the relevant .jar/.war archive).
Mechanism in Java5/Java6
In order to create a Java class at runtime without Java source code, the use of a custom ClassLoader is required, along with a bytecode manipulation framework (such as ASM or some other library).
Java classloaders form an instance-hierarchy at run-time, with the system (Bootstrap, Extension and System) class loaders
strictly controlled by the JVM. Once an application is launched (via an Application loader), a new loader MyCustomClassLoader
can
be added to the chain.
The basic implementation pattern is as follows - in the constructor, the new instance of MyCustomClassLoader
is added to the
runtime instance-hierarchy by calling super
with the parent loader.
public class MyCustomClassLoader extends ClassLoader { public MyCustomClassLoader (ClassLoader parent) { super(parent); } @Override protected Class<?> findClass(String className) throws ClassNotFoundException { if (some_condition) { try { byte[] bytes = use_framework_to_generate_bytecode(); return defineClass(className, bytes, 0, bytes.length); } catch (ClassFormatError cfe) { throw new ClassNotFoundException(className, cfe); } } return super.findClass(className); } }
The findClass
method is overridden so that if some condition is met, the bytecode for the Class className
is
generated; otherwise, the call is delegated up the instance-hierarchy to search for the class. The implementation hierarchy is responsible
for maintaining a cache of classes, as well as any resources that have been loaded (XML descriptor files, image files, etc). Thus, an instance
of MyCustomClassLoader
behaves as a 'proper' class loader in all cases, with the additional capability that non-existent
classes can be built/found without their corresponding .class files being on the JVM's classpath.
NB: it is important to note that two separate instances of MyCustomClassLoader
can generate two classes
for className
. Even though the bytecode is identical, the classes are distinct. When running under a Java EE™ container
(or an OSGi environment), the class loader hierarchy may be much more complicated, but the point remains - a custom classloader is
required to build classes at runtime; the custom classloader is part of a chain of loaders and each dynamic class is distinct.
It is thus incumbent upon the designer to ensure that the correct loader is used throughout the lifecycle of the application.