Question

My Java app displays its version number in a few places, so I store that as a public final variable in the app’s main class. Is there any way to access that in my Ant build tasks? In the spirit of automating the build process, I’d like Ant to automatically name the zipped distributable with the current version number, but I’m not sure how. I’m guessing it would look something like…

<target name="createZip" depends="build">
    <zip destfile="../dist/MyApp_${MyApp.version}.zip">
        <fileset dir="../dist">
            <include name="**/MyApp"/>
        </fileset>
    </zip>
</target>

I’m just not sure what to put in the place of ${MyApp.version}. I realize I could put this in the build properties file, but it would be a lot more convenient to be able to pull it straight from the class file I’m already storing it in.

Answer 1

I don’t think there are any built in Ant tasks that do what you want. However, you could roll your own, taking advantage of the extensible nature of Ant.

I’ve hacked up a really (and I do mean really) dirty example you can use as a spring board to a proper Task.

package q1015732;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.lang.reflect.Field;

import org.apache.tools.ant.BuildException;
import org.apache.tools.ant.Task;

/**
 * Custom ant task that binds a property name to a static member of a class file.
 * 
 * @author Montrose
 */
public class BindPropertyTask extends Task{
    private String _classFile = null;
    private String _fieldName = null;
    private String _propName = null;

    /**
     * Set the field name of the class.
     * 
     * @param fieldName
     */
    public void setFieldName(String fieldName){
        _fieldName = fieldName;
    }

    /**
     * The class file.
     * @param classFile
     */
    public void setClassFile(String classFile){
        _classFile = classFile;
    }

    /**
     * The property name to bind
     * @param propName
     */
    public void setProperty(String propName)
    {
        _propName = propName;
    }

    /**
     * Load the class file, and grab the value of the field.
     * 
     * Throws exceptions if classfile, fieldname, or property have not been set.
     * 
     * Throws more execeptions if classfile does not exist, the field does not exist, or the field is not static.
     */
    public void execute() throws BuildException{
        if(_classFile == null) throw new BuildException("ClassFile is a required attribute");
        if(_fieldName == null) throw new BuildException("FieldName is a required attribute");
        if(_propName == null)  throw new BuildException("Property is  required attribute");

        CustomLoader loader = new CustomLoader();
        Class toInspect = null;
        Field toBind = null;
        Object value = null;

        try {
            toInspect = loader.loadClass(new FileInputStream(_classFile));
        } catch (Exception e) {
            throw new BuildException("Couldn't load class ["+e.getMessage()+"], in ["+(new File(_classFile).getAbsolutePath())+"]");
        }

        try{
            toBind = toInspect.getField(_fieldName);
        }catch(NoSuchFieldException e){
            throw new BuildException("Couldn't find field, '"+_fieldName+"'");
        }

        //Let us bind to private/protected/package-private fields
        toBind.setAccessible(true);

        try{
            value = toBind.get(null);
        }catch(NullPointerException e){
            throw new BuildException("Field is not static");
        } catch (Exception e) {
            throw new BuildException("Unable to access field ["+e.getMessage()+"]");
        }

        if(value != null)
            this.getProject().setProperty(_propName, value.toString());
        else
            this.getProject().setProperty(_propName, null);
    }

    /**
     * Custom class loader, for loading a class directly from a file.
     * 
     * This is hacky and relies on deprecated methods, be wary.
     * 
     * @author Montrose
     */
    class CustomLoader extends ClassLoader{
        public CustomLoader(){
            super(ClassLoader.getSystemClassLoader());
        }

        /**
         * Warning, here be (deprecated) dragons.
         * @param in
         * @return
         * @throws Exception
         */
        @SuppressWarnings("deprecation")
        public Class loadClass(InputStream in) throws Exception{
            byte[] classData = loadData(in);
            return this.defineClass(classData, 0, classData.length);
        }

        private byte[] loadData(InputStream in) throws Exception{
            byte[] buffer = new byte[1024];
            ByteArrayOutputStream out = new ByteArrayOutputStream();
            int i;


            while((i = in.read(buffer)) != -1){
                out.write(buffer, 0, i);
            }

            return out.toByteArray();
        }
    }
}

An example build file using this task:

<project name="q1015732">

<target name="test">
<taskdef name="static_bind" classname="q1015732.BindPropertyTask" />

<static_bind fieldname="StringBind" classfile="C:\Users\Montrose\workspace\StackOverflow Questions\q1015732\test\DummyMain.class" property="string.value" />
<static_bind fieldname="IntBind"    classfile="C:\Users\Montrose\workspace\StackOverflow Questions\q1015732\test\DummyMain.class" property="int.value" />
<static_bind fieldname="DoubleBind" classfile="C:\Users\Montrose\workspace\StackOverflow Questions\q1015732\test\DummyMain.class" property="double.value" />

<echo message="StringBind: ${string.value}" />
<echo message="IntBind:    ${int.value}" />
<echo message="DoubleBind: ${double.value}" />

</target>

</project>

DummyMain.java:

package q1015732.test;

public class DummyMain {
    public static String StringBind = "I'm a String!";
    public static int    IntBind    = 1024;
    public static double DoubleBind = 3.14159;
}

The result of a build using the Ant file:

test:
[echo] StringBind: I’m a String!
[echo] IntBind: 1024
[echo] DoubleBind: 3.14159
BUILD SUCCESSFUL

There are a number of random problems with this task: it relies on deprecated methods, it takes files instead of class names, and the error reporting leaves a bit to be desired. However, you should get the gist of what is needed for a custom task that solves your problem.