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Difference between revisions of "EclipseLink/Development/Dynamic/def1"
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===== Mechanism in Java5/Java6 ===== | ===== Mechanism in Java5/Java6 ===== | ||
− | In order to create a Java class at runtime without | + | 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 |
− | along with a bytecode manipulation | + | manipulation framework (such as [http://asm.objectweb.org ASM] or some other library [http://www.java-source.net/open-source/bytecode-libraries framework]). |
− | Java classloaders form an inheritance at run-time, with the | + | |
+ | Java classloaders form an inheritance-chain at run-time, with the system (<b>Bootstrap</b>, <b>Extension</b> and <b>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. | ||
+ | |||
+ | <br /> | ||
[[Image:custom_classloader.gif]] | [[Image:custom_classloader.gif]] | ||
Revision as of 15:30, 31 August 2009
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
. AnonymousClassLoader
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.