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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 framework).
Java classloaders form an instance-chain 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-chain 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 implementation hierarchy which at runtime searches for the class up the instance
hierarchy. The implementation hierarchy is responsible for maintaining a cache of both found and generated classes, as well as any
resources that have been loaded (XML descriptor files, image files, etc).
Mechanism in Java7
Future
JSR-292 (Supporting Dynamically Typed Languages on the Java ™ Platform) introduces a new type of classloader,
java.dyn.AnonymousClassLoader
. The AnonymousClassLoader
class is designed to solve two problems:
- Generating many classes with similar bytecode and only minor changes is very inefficient, wasting a lot of precious memory.
- Generated bytecode must be contained in a class, which must be contained in a ClassLoader, which keeps a hard reference to the class; as a result, to make even one byte of bytecode garbage-collectable, it must be wrapped in its own class and its own classloader.
First, classes loaded by AnonymousClassLoader
are not given full-fledged symbolic names in the global symbol tables; they're given rough numeric identifiers. They are effectively anonymized, allowing much more freedom to generate them at will, since naming conflicts essentially do not happen.
Second, the classes are loaded without a parent ClassLoader, so there's no overprotective mother keeping them on a short leash. When the last normal references to the class disappear, it's eligible for garbage collection like any other object.
Third, it provides a mechanism whereby an existing class can be loaded and slightly modified, producing a new class with those modifications but sharing the rest of its structure and data.