Section 7.1. Executing Java Code

7.1. Executing Java Code

The java task is part of Ant's core functionality; it executes a Java class in the current JVM, or forks another JVM and runs the class in the new JVM. You can recover the exit code of the Java class and stop the build if the build results you're testing create an error.

Here's an example using this task. Say you have this code,, which reads what the user enters on the command line and displays it:

public class Project  {     public static void main(String args[])      {         System.out.println("You said: " + args[0]);             System.exit(0);     } }

After compiling this code you can run it with the java task by setting up the classpath with a classpath element and passing a command-line argument, "OK", in a nested arg element. The build file appears in Example 7-1.

Example 7-1. Using the java task (ch07/java/build.xml)
<?xml version="1.0" ?> <project default="main">     <property name="src" location="source" />     <property name="output" location="bin" />     <property environment="env" />     <target name="main" depends="init, compile, run">         <echo>             Building and running....         </echo>     </target>        <target name="init">         <mkdir dir="${output}" />     </target>        <target name="compile">         <javac srcdir="${src}" destdir="${output}" />     </target>        <target name="run" failonerror="true">         <java classname="Project"             fork="true" >             <classpath>                <pathelement location="${output}"/>             </classpath>             <arg value="OK" />         </java>     </target> </project>

Here's what you see when you run this build file; the code ran without problem and recovered the command-line argument passed to it:

%ant Buildfile: build.xml init:     [mkdir] Created dir: /home/steven/ch07/bin compile:     [javac] Compiling 1 source file to /home/steven/ch07/bin run:      [java] You said: OK main:      [echo]      [echo]             Building and running....      [echo] BUILD SUCCESSFUL Total time: 4 seconds

The many attributes for this task appear in Table 7-1.

If things go wrong when you run this task, there may be a conflict with the current JVM, which is running Ant. In that case, set fork="true" to use a new JVM.

Table 7-1. The java task's attributes






Specifies whether you want to append to output and error files.




Deprecated. Use nested arg elements. Specifies the arguments for the class that you want to run.




Specifies the Java class you want to run.

One of either jar or classname



Specifies the classpath you want to use when the class is run.




Specifies the classpath you want to use, as a reference, when the class is run.




Specifies the directory where you want to run Java.




Specifies the file where standard error output should be stored.




Specifies the name of a property where you want to store errors.




Specifies the build should be stopped if the task encounters errors.




Specifies you want to run the class in a forked JVM.




Specifies the file where the task should take input to run the class with.




Specifies a string holding the input stream for the class to run.




Specifies the location of the .jar file to run. The .jar file must have a Main-Class entry in the manifest.

jar or classname



Specifies the command used to start Java.




Deprecated. Use nested jvmarg elements. Specifies arguments to pass to the forked Java Virtual Machine.




Specifies you want to send error output to Ant's log.




Specifies the maximum amount of memory you want to give a forked JVM.




Specifies old environment variables should not be passed as new environment variables to a forked JVM.




Specifies the name of a file in which to store the output.




Specifies the name of a property in which you want the output of the task to be placed.




Specifies the name of the property that you want to hold the return code. Use this only if failonerror is false and if fork is true.




Specifies you want to spawn a new process in which to run the class. To use this attribute, set fork to true.




Specifies you want the task to quit if it doesn't finish in the given time. Set the time in milliseconds. You should only use this if fork is true.



The java task supports a number of nested elements, many of which are the same as the javac task. You can use arg elements to pass arguments to Java and jvmarg elements to specify arguments to a forked JVM. Nested sysproperty elements specify system properties required by the class you're running. As of Ant 1.6, you can use syspropertyset elements, which specify a set of properties to be used as system properties.

The java task supports nested classpath elements, which you can use to specify a classpath to use when Java runs, and supports as bootclasspath elements (since Ant 1.6) to set the location of bootstrap class files. You can use env elements (see Table 7-3) to specify environment variables to pass to the forked JVM and nested permissions elements. As with the javac task, permissions represents a set of security permissions granted to the code in the JVM where Ant is running. Since Ant 1.6, you can use nested assertions elements to support Java 1.4 assertions.

7.1.1. Handling Errors and Return Codes

By default, the return code of the java task is ignored. If you want to check the return code, you can set the resultproperty attribute to the name of a property and have the result code assigned to it. For example, say your code returned a non-zero value:

public class Project  {     public static void main(String args[])      {         System.out.println("You said: " + args[0]);             System.exit(1);     } }

You can test the return code from a forked JVM and explicitly fail unless it's 0 this way:

    <target name="run">         <java classname="Project"             fork="true" resultproperty="return.code">             <classpath>                <pathelement location="${output}"/>             </classpath>             <arg value="OK" />         </java>         <condition property="problem">             <not>                 <equals arg1="${return.code}" arg2="0"/>             </not>         </condition>         <fail if="problem" message="Failed: ${return.code}" />     </target>

Here's the result. The java task indicates a nonzero return code and the build was terminated by the fail task:

%ant build.xml Buildfile: build.xml init:     [mkdir] Created dir: /home/steven/ch07/bin compile:     [javac] Compiling 1 source file to /home/steven/ch07/bin run:      [java] You said: OK      [java] Java Result: 1 BUILD FAILED /home/steven/ch07/build2.xml:35: Failed: 1 Total time: 4 seconds

You can set failonerror="true" in the java task, in which case the only possible value for resultproperty is 0, or the build will terminate. That's how the example in the previous topic was written:

    <target name="run" failonerror="true">         <java classname="Project"             fork="true" >             <classpath>                <pathelement location="${output}"/>             </classpath>             <arg value="OK" />         </java>     </target>

If failonerror="false" and fork="false", the java task must return a value of 0 or the build will exit because the code was run by the build JVM.

Making a build fail if there's an error when you run the build's output is a perfect way to test the results of a build; if the output doesn't run as it should, there's no sense in deploying it. Setting failonerror to true in the java task ensures your build will halt before deployment if the results don't work.

Here's another example using the java task, which forks a JVM and runs a .jar file in 512 Megabytes of memory, using the entry point indicated by the manifest:

<java jar="${bin}/connect.jar"     fork="true"     failonerror="true"     maxmemory="512m"     >     <arg value="-q"/>     <classpath>         <pathelement location="${bin}/connect.jar"/>         <pathelement path="${java.class.path}"/>     </classpath> </java>

This example passes on a system property and an argument to the JVM:

<java classname="Project.main" fork="true" >     <sysproperty key="DEBUG" value="true"/>     <arg value="-z"/>     <jvmarg value="-enableassertions"/>     <classpath>         <pathelement location="${bin}/**"/>     </classpath> </java>

As you can see, there are a great many options when running Java code.

    Ant. The Definitive Guide
    Ant: The Definitive Guide, 2nd Edition
    ISBN: 0596006098
    EAN: 2147483647
    Year: 2003
    Pages: 115
    Authors: Steve Holzner

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