DataNucleus Enhancer

As is described in the ByteCode Enhancement guide, DataNucleus utilises the common technique of byte-code manipulation to make your normal Java classes "persistable". The mechanism provided by DataNucleus is to use an "enhancer" process to perform this manipulation before you use your classes at runtime. The process is very quick and easy.

How to use the DataNucleus Enhancer depends on what environment you are using. Below are some typical examples.


DataNucleus provides a JAR containing the Enhancer ( datanucleus-enhancer.jar ). If you are building your application manually and want to enhance your classes you follow the instructions in this section. You invoke the enhancer as follows

java -cp classpath  org.datanucleus.enhancer.DataNucleusEnhancer [options] [jdo-files] [class-files]
    where options can be
        -pu {persistence-unit-name} : Name of a "persistence-unit" to enhance the classes for
        -d {target-dir-name} : Write the enhanced classes to the specified directory
        -api {api-name} : Name of the API we are enhancing for (JDO, JPA). Default is JDO
        -enhancerName {name} : Name of the ClassEnhancer to use. Options ASM
        -checkonly : Just check the classes for enhancement status
        -v : verbose output
        -q : quiet mode (no output, overrides verbose flag too)
        -generatePK {flag} : generate any PK classes where needed 
                             ({flag} should be true or false - default=true)
        -generateConstructor {flag} : generate default constructor where needed 
                             ({flag} should be true or false - default=true)

    where classpath must contain the following
        datanucleus-api-jdo.jar/datanucleus-api-jpa.jar (if using JDO or JPA respectively)
        log4j.jar (optional)
        jpa-api.jar (optional - if using JPA)
        your classes
        your meta-data files

The input to the enhancer should be either a set of MetaData/class files or the name of the "persistence-unit" to enhance. In the first option, if any classes have annotations then they must be specified. All classes and MetaData files should be in the CLASSPATH when enhancing. To give an example of how you would invoke the enhancer

Linux/Unix :
java -cp target/classes:lib/datanucleus-enhancer.jar:lib/datanucleus-core.jar:lib/jdo-api.jar:

Windows :
java -cp target\classes;lib\datanucleus-enhancer.jar;lib\datanucleus-core.jar;lib\jdo-api.jar;
     org.datanucleus.enhancer.DataNucleusEnhancer -v

[should all be on same line. Shown like this for clarity]

So you pass in your JDO MetaData files (and/or the class files wihich use annotations) as the final argument(s) in the list, and include the respective JAR's in the classpath (-cp). The enhancer responds as follows

DataNucleus Enhancer (version 3.0.0.m1) : Enhancement of classes

DataNucleus Enhancer : Classpath
>>  /home/andy/work/myproject//target/classes
>>  /home/andy/work/myproject/lib/log4j.jar
>>  /home/andy/work/myproject/lib/jdo-api.jar
>>  /home/andy/work/myproject/lib/datanucleus-core.jar
>>  /home/andy/work/myproject/lib/datanucleus-api-jdo.jar
>>  /home/andy/work/myproject/lib/datanucleus-enhancer.jar
>>  /home/andy/work/myproject/lib/asm.jar

DataNucleus Enhancer : Using ClassEnhancer "asm" for API "JDO"

DataNucleus Enhancer : Input Files
>>  /home/andy/work/myproject/target/classes/org/mydomain/mypackage1/package.jdo
>>  /home/andy/work/myproject/target/classes/org/mydomain/mypackage2/package.jdo

Processing class "org.mydomain.mypackage1.Pack"
ENHANCED: org.mydomain.mypackage1.Pack
Processing class "org.mydomain.mypackage1.Card"
ENHANCED: org.mydomain.mypackage1.Card
Processing class "org.mydomain.mypackage2.Pack"
ENHANCED: org.mydomain.mypackage2.Pack
Processing class "org.mydomain.mypackage2.Card"
ENHANCED: org.mydomain.mypackage2.Card
DataNucleus Enhancer completed with success for 4 classes. Timings : input=422 ms, enhance=490 ms, total=912 ms.
     ... Consult the log for full details

If you have errors here relating to "Log4J" then you must fix these first. If you receive no output about which class was ENHANCED then you should look in the DataNucleus enhancer log for errors. The enhancer performs much error checking on the validity of the passed MetaData and the majority of errors are caught at this point. You can also use the DataNucleus Enhancer to check whether classes are enhanced. To invoke the enhancer in this mode you specify the checkonly flag. This will return a list of the classes, stating whether each class is enhanced for persistence under JDO or not. The classes need to be in the CLASSPATH (Please note that a CLASSPATH should contain a set of JAR's, and a set of directories. It should NOT explictly include class files, and should NOT include parts of the package names. If in doubt please consult a Java book).


Maven2 operates from a series of plugins. There is a DataNucleus plugin for Maven2 that allows enhancement of classes. Go to the Download section of the website and download this. Once you have the M2 plugin, you then need to set any properties for the plugin in your pom.xml file. If you are using annotations then you'll need to add *.class to "mappingIncludes" for example. Some properties that you may need to change are below

Property Default Description
metadataDirectory ${} Directory to use for enhancement files (classes/mappings)
metadataIncludes **/*.jdo, **/*.class Fileset to include for enhancement
metadataExcludes Fileset to exclude for enhancement
persistenceUnitName Name of the persistence-unit to enhance
log4jConfiguration Config file location for Log4J (if using it)
jdkLogConfiguration Config file location for JDK1.4 logging (if using it)
api JDO API to enhance to (JDO, JPA)
verbose false Verbose output?
quiet false No output?
targetDirectory Where the enhanced classes are written (default is to overwrite them)
fork true Whether to fork the enhancer process
generatePK true Generate a PK class (of name {MyClass}_PK) for cases where there are multiple PK fields yet no PK class is defined. (from M2 plugin v3.0.1)
generateConstructor true Generate a default constructor if not defined for the class being enhanced. (from M2 plugin v3.0.1)
detachListener false Whether to enhance classes to make use of a detach listener for attempts to access an undetached field. (from M2 plugin v3.0.1)

You then run the Maven2 DataNucleus plugin, as follows

mvn datanucleus:enhance

This will enhance all classes found that correspond to the classes defined in the JDO files in your source tree. If you want to check the current status of enhancement you can also type

mvn datanucleus:enhance-check

Or alternatively, you could add the following to your POM


So you then get auto-enhancement after each compile


Ant provides a powerful framework for performing tasks. DataNucleus provides an Ant task to enhance classes. DataNucleus provides a JAR containing the Enhancer ( datanucleus-enhancer.jar ). You need to make sure that the datanucleus-enhancer.jar , datanucleus-core.jar , datanucleus-api-jdo.jar / datanucleus-api-jpa.jar (if using JDO or JPA respectively), asm.jar , log4j.jar and jdo-api.jar are in your CLASSPATH. Additionally, if using JPA, you will need jpa-api.jar in the CLASSPATH too. In the DataNucleus Enhancer Ant task, the following parameters are available

Parameter Description values
dir Optional. Directory containing the JDO files to use for enhancing. Uses ant build file directory if the parameter is not specified.
destination Optional. Defining a directory where enhanced classes will be written. If omitted, the original classes are updated.
api Optional. Defines the API to be used when enhancing JDO , JPA
enhancerName Optional. Defines the ClassEnhancer to use when enhancing. ASM
persistenceUnit Optional. Defines the "persistence-unit" to enhance.
checkonly Whether to just check the classes for enhancement status. Will respond for each class with "ENHANCED" or "NOT ENHANCED". This will disable the enhancement process and just perform these checks. true, false
verbose Whether to have verbose output. true, false
quiet Whether to have no output. true, false
generatePK Whether to generate PK classes as required. true , false
generateConstructor Whether to generate a default constructor as required. true , false
filesuffixes Optional. Suffixes to accept for the input files. The Enhancer Ant Task will scan for the files having these suffixes under the directory specified by dir option. The value can include comma-separated list of suffixes. If using annotations you can have "class" included as a valid suffix here or use the fileset . jdo
fileset Optional. Defines the files to accept as the input files. Fileset enables finer control to which classes / metadata files are accepted to enhanced. If one or more files are found in the fileset, the Enhancer Ant Task will not scan for additional files defined by the option filesuffixes . For more information on defining a fileset, see Apache FileSet Manual.
if Optional. The name of a property that must be set in order to the Enhancer Ant Task to execute.

The enhancer task extends the Apache Ant Java task, thus all parameters available to the Java task are also available to the enhancer task.

So you could define something like the following, setting up the parameters enhancer.classpath , jdo.file.dir , and log4j.config.file to suit your situation (the jdo.file.dir is a directory containing the JDO files defining the classes to be enhanced). The classes specified by the XML Meta-Data files, together with the XML Meta-Data files must be in the CLASSPATH (Please note that a CLASSPATH should contain a set of JAR's, and a set of directories. It should NOT explictly include class files, and should NOT include parts of the package names. If in doubt please consult a Java book).

<target name="enhance" description="DataNucleus enhancement">
    <taskdef name="datanucleusenhancer" classpathref="enhancer.classpath" 
                classname="" />

    <datanucleusenhancer classpathref="enhancer.classpath"
        dir="${jdo.file.dir}" failonerror="true" verbose="true">
        <jvmarg line="-Dlog4j.configuration=${log4j.config.file}"/>

You can also define the files to be enhanced using a fileset . When a fileset is defined, the Enhancer Ant Task will not scan for additional files, and the option filesuffixes is ignored.

<target name="enhance" description="DataNucleus enhancement">
    <taskdef name="datanucleusenhancer" classpathref="enhancer.classpath" 
                classname="" />

        dir="${jdo.file.dir}" failonerror="true" verbose="true">
        <fileset dir="${classes.dir}">
            <include name="**/*.jdo"/>
            <include name="**/acme/annotated/persistentclasses/*.class"/>
            <path refid="enhancer.classpath"/>

You can disable the enhancement execution upon the existence of a property with the usage of the if parameter.

<target name="enhance" description="DataNucleus enhancement">
    <taskdef name="datanucleusenhancer" classpathref="enhancer.classpath" 
                classname="" if="aPropertyName"/>

    <datanucleusenhancer classpathref="enhancer.classpath"
        dir="${jdo.file.dir}" failonerror="true" verbose="true">
        <jvmarg line="-Dlog4j.configuration=${log4j.config.file}"/>
Runtime Enhancement

Enhancement of persistent classes at runtime is possible when using JRE 1.5 or superior versions. Runtime Enhancement requires the following runtime dependencies: ASM, and DataNucleus Core libraries. To enable runtime enhancement, the javaagent option must be set in the java command line. For example,

java -javaagent:datanucleus-enhancer.jar Main

The statement above will mean that all classes, when being loaded, will be processed by the ClassFileTransformer (except class in packages "java.*", "javax.*", "org.datanucleus.*"). This means that it can be slow since the MetaData search algorithm will be utilised for each. To speed this up you can specify an argument to that command specifying the names of package(s) that should be processed (and all others will be ignored). Like this

java -javaagent:datanucleus-enhancer.jar=mydomain.mypackage1,mydomain.mypackage2 Main

so in this case only classes being loaded that are in mydomain.mypackage1 and mydomain.mypackage2 will be attempted to be enhanced.

For JPA, it would look like this

java -javaagent:datanucleus-enhancer.jar=-api=JPA,mydomain.mypackage1,mydomain.mypackage2 Main

Please take care over the following when using runtime enhancement

  • When you have a class with a field of another persistable type make sure that you mark that field as "persistent" (@Persistent, or in XML) since with runtime enhancement at that point the related class is likely not yet enhanced so will likely not be marked as persistent otherwise. Be explicit
  • If the agent jar is not found make sure it is specified with an absolute path.

Parameter Description values
api Optional. Name of the API we are enhancing for. JDO | JPA

Programmatic API

You could alternatively programmatively enhance classes from within your application. This is done as follows

import javax.jdo.JDOEnhancer;

JDOEnhancer enhancer = JDOHelper.getEnhancer();

This will look in META-INF/persistence.xml and enhance all classes defined by that unit.


DataNucleus provides a JAR containing the Enhancer ( datanucleus-enhancer.jar ). From JDK 1.6 and forward you can automatically enhance your classes when compiling your classes, as long as your classes have annotations (since this is an AnnotationProcessor ). To do this you need to

  1. Use JDK1.6+
  2. Add datanucleus-enhancer.jar , datanucleus-core.jar , datanucleus-api-jdo.jar / datanucleus-api-jpa.jar (depends if using JDO or JPA) jdo-api.jar and asm.jar to the compiler classpath
  3. Set the processor compiler argument to org.datanucleus.enhancer.EnhancerProcessor . In Maven2 you would add the following to your POM
  4. Classes will be enhanced when you compile
Note that this works by waiting a period of time until all classes will have been compiled before invoking the enhancement step. We advise against using it, and better to use post-compile directly

If using Eclipse you would need to use an Ant build.xml because the Eclipse compiler doesn't support specification of the processor compiler argument. However with Eclipse you can always just use the DataNucleus Eclipse plugin!

To enhance classes for JPA (rather than JDO) using this method you need to specify -AenhanceAPI=JPA as a compiler argument.