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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 ClassLoaderer is required, along with a bytecode manipulation framework (such as ASM or some other library framework).
Java classloaders form an inheritance-chain at run-time, with the system (Bootstrap, Extension and System) class loaders strictly controlled by the JVM. Once an application is launched (adds an Application loader), new loaders can be added to the chain.
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.