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Difference between revisions of "EclipseLink/Development/Dynamic/def1"
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===== Dynamic Persistence ===== | ===== Dynamic Persistence ===== | ||
− | Dynamic Persistence is defined as the ability to create a persistent entity class and use it within an application without the Java class existing (no .java or .class files). This is typically accomplished by creating a class on-the-fly with a custom ClassLoader that uses a bytecode manipulation library (such as [http://asm.objectweb.org ASM] or some other | + | Dynamic Persistence is defined as the ability to create a persistent entity class and use it within an application without the Java class existing (no .java or .class files). This is typically accomplished by creating a class on-the-fly with a custom ClassLoader that uses a bytecode manipulation library (such as [http://asm.objectweb.org ASM] or some other [http://www.java-source.net/open-source/bytecode-libraries framework]). |
Dynamic Persistence is - in essence - a <i>trick</i> with ClassLoaders. EclipseLink's meta-data describes a persistent entity in terms of a <b>Descriptor</b> which owns (one or more) <b>Mappings</b> that represent a class' member fields. The class' info is string-based - for instance in code: | Dynamic Persistence is - in essence - a <i>trick</i> with ClassLoaders. EclipseLink's meta-data describes a persistent entity in terms of a <b>Descriptor</b> which owns (one or more) <b>Mappings</b> that represent a class' member fields. The class' info is string-based - for instance in code: |
Revision as of 16:28, 27 August 2009
Dynamic Persistence
Dynamic Persistence is defined as the ability to create a persistent entity class and use it within an application without the Java class existing (no .java or .class files). This is typically accomplished by creating a class on-the-fly with a custom ClassLoader that uses a bytecode manipulation library (such as ASM or some other framework).
Dynamic Persistence is - in essence - a trick with ClassLoaders. EclipseLink's meta-data describes a persistent entity in terms of a Descriptor which owns (one or more) Mappings that represent a class' member fields. The class' info is string-based - for instance in code:
RelationalDescriptor addressDescriptor = new RelationalDescriptor(); addressDescriptor.setAlias("address"); addressDescriptor.setJavaClassName("com.foo.bar.Address");
or XML:
<?xml version="1.0" encoding="UTF-8"?> <object-persistence version="Eclipse Persistence Services - {some version}" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:eclipselink="http://www.eclipse.org/eclipselink/xsds/persistence" > <name>relationships</name> <class-mapping-descriptors> <class-mapping-descriptor xsi:type="xml-class-mapping-descriptor"> <class>com.foo.bar.Address</class> <alias>address</alias>
Because almost any value can be specified for the <class> (within the limits of Java naming requirements), there is the possibility that at runtime the class cannot be found. The trick is to provide a custom ClassLoader that upon detecting that com.foo.bar.Address does not exist, uses a bytecode manipulation library to create a class that agrees with the meta-data - in terms of the Mappings, their types, etc.