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copied!<p>By default, <code><javac></code> runs in-process with Ant. It is a general limitation of Java that you can't adjust a JVM process' <code>Xms</code> and <code>Xmx</code> once that JVM process has launched. So, the error message that you are seeing is the software rejecting your attempt to violate this principle (using an unhelpful, unfriendly error message.)</p> <p>If, however, you specify the attribute <code>fork="true"</code> on the <code><javac></code> tag you <em>will</em> be able to specify a new <code>Xms</code> and <code>Xms</code>. This is because <code>fork</code> instructs Ant to launch a new JVM subprocess in which to run <code>javac</code>. Because the JVM process is new, it gives Ant an acceptable opportunity to specify <code>Xms</code> and <code>Xmx</code> for it.</p> <hr> <p>You might try something like this:</p> <pre><code><project name="project" default="all" basedir="[yourvalue]"> <target name="all"> <javac srcdir="[yourvalue]" destdir="[yourvalue]" fork="true">  <compilerarg line="-J-Xms128m -J-Xmx512m" /> </javac> </target> </project> </code></pre> <p>(Notice I am using <code>compilerarg line=""</code> rather than <code>compilerarg value=""</code>. The <code>line</code> attribute lets you specify multiple space-separated arguments. The <code>value</code> attribute is for passing a single argument.)</p> <hr> <p>Ant will wait for the forked <code><javac></code> to exit, which happens after the <code>javac</code> process finishes its work (i.e. compiling). Ant then continues running the build script inside its own original JVM process. Ant will check if the forked <code>javac</code> failed or succeeded, and take the usual actions based on this information.</p> <hr> <p><strong>Performance</strong></p> <p>It's usually more performant to <em>not</em> fork <code>javac</code>, and instead simply tune the relevant memory settings for the initial Ant JVM overall. This is often (but not always) the best choice because launching a separate JVM is usually slower and takes more memory than simply allowing <code>javac</code> to run in-process.</p> <p>If you are using the Ant-provided <code>ant.bat</code> or <code>ant.sh</code> to launch Ant, an easy way to tune Ant's <code>Xms</code> and <code>Xmx</code> is to define the environment variable ANT_OPTS to contain the arguments you want. There many ways to set environment variables, but you could just edit <code>ant.bat</code>:</p> <pre><code>set ANT_OPTS=-Xms128m -Xmx512m </code></pre>

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