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Difference between revisions of "EclipseLink/Development/Dynamic/def1"
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<css>.source-xml {padding:4px;border:1px solid black;}</css><css>.source-java5 {padding:4px;border:1px solid black;}</css> | <css>.source-xml {padding:4px;border:1px solid black;}</css><css>.source-java5 {padding:4px;border:1px solid black;}</css> | ||
===== 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 <tt>.java</tt> or <tt>.class</tt> files | + | 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 <tt>.java</tt> or <tt>.class</tt> files). |
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: | ||
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<alias>address</alias> | <alias>address</alias> | ||
</source> | </source> | ||
− | Because almost any value can be specified for the <b><class></b> (within the limits of Java naming | + | Because almost any value can be specified for the <b><class></b> (within the limits of Java naming conventions), there is the possibility that the class cannot be found. The <i>trick</i> then is to provide a custom ClassLoader that upon detecting that <tt>com.foo.bar.Address</tt> does not exist, 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]) to <b>create</b> a class that agrees with the meta-data - in terms of the number of Mappings, their types, etc. |
Revision as of 16:33, 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).
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: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 conventions), there is the possibility that the class cannot be found. The trick then is to provide a custom ClassLoader that upon detecting that com.foo.bar.Address does not exist, uses a bytecode manipulation library (such as ASM or some other framework) to create a class that agrees with the meta-data - in terms of the number of Mappings, their types, etc.