First Step: Source and Target Configurations

The first step to supporting JDKs older than 1.5 in your build is already done for you: setting the source and target API versions in the configuration for the compiler plugin. Since JDK 1.5+ can still generate class files that are compliant with older JDKs, using this default compiler-plugin setting will render project artifacts that are technically compatible with JVMs older than 1.5.

This is great, until someone sneaks a String.contains( "foo" ) into the codebase. If that happens, your project will still build successfully, and generate 1.4-compatible binaries. But try to execute your code in a 1.4 JVM, and you'll understand why the compiler configuration alone isn't enough. To construct a build process targeting JVM 1.4 when the build tool requires JVM 1.5+, while still ensuring the result of that build is safe to run on the 1.4 JVM, we need to use a tool that can verify the compatibility of our project code against a given Java API specification version.

Enter the Animal Sniffer

Fortunately, there is a project at dev.java.net called Animal Sniffer that does exactly what we need. This project can verify the compatibility of a codebase against a whole range of Java APIs. Even better, it provides a Maven 2.x plugin to give users an easy way to integrate these checks into their build. To verify that your codebase complies with JDK 1.4 API, you simply tell the animal sniffer plugin to use:

<signature>
  <groupId>org.jvnet.animal-sniffer</groupId>
  <artifactId>java1.4</artifactId>
  <version>1.0</version>
</signature>

Sample Configuration

Putting it all together, the following example shows how you can be certain your project will run on the 1.4 JVM:

<build>
  <pluginManagement>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>2.0.2</version>
        <configuration>
          <source>1.4</source>
          <target>1.4</target>
        </configuration>
      </plugin>
    </plugins>
  </pluginManagement>

  <plugins>
    <plugin>
      <groupId>org.jvnet</groupId>
      <artifactId>animal-sniffer</artifactId>
      <version>1.2</version>
      <executions>
        <execution>
          <id>check-java-version</id>
          <phase>compile</phase>
          <goals>
            <goal>check</goal>
          </goals>
          <configuration>
            <signature>
              <groupId>org.jvnet.animal-sniffer</groupId>
              <artifactId>java1.4</artifactId>
              <version>1.0</version>
            </signature>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>