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    <p>To get the object size using instrumentation, it is necessary to load an agent into the jvm, here is agent code and manifest</p> <p>Agent-MANIFEST.MF</p> <pre><code>Premain-Class: mypackage.Agent Agent-Class: mypackage.Agent Can-Retransform-Classes: true </code></pre> <p>Agent.java</p> <pre><code>/* Agent.java javac -cp ".:$JAVA_HOME/lib/tools.jar" -d . Agent.java Test.java &amp;&amp; \ jar cfm Agent.jar Agent-MANIFEST.MF mypackage/Agent.class */ package mypackage; import java.lang.instrument.Instrumentation; import java.lang.instrument.ClassFileTransformer; import java.lang.instrument.IllegalClassFormatException; import java.security.ProtectionDomain; public class Agent implements ClassFileTransformer { public static Instrumentation inst; public static void premain(String agentArgs, Instrumentation inst) { Agent.inst = inst; } public static void agentmain(String agentArgs, Instrumentation inst) { Agent.inst = inst; } public byte[] transform(ClassLoader loader, String className, Class redefiningClass, ProtectionDomain domain, byte[] bytes) throws IllegalClassFormatException { /* returning null means we don't want to change a thing */ return null; } } </code></pre> <p>the agent above allows you this</p> <p><code>GetObjectSizeTest.java</code></p> <pre><code>package mypackage; import java.io.IOException; import java.io.PrintWriter; import java.util.Enumeration; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public final class GetObjectSizeTest extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/html"); PrintWriter writer = response.getWriter(); writer.println("&lt;html&gt;"); writer.println("&lt;body bgcolor=white&gt;"); writer.println("&lt;p&gt;The size of System.in is " + Agent.inst.getObjectSize(System.in) + "&lt;/p&gt;"); writer.println("&lt;/body&gt;"); writer.println("&lt;/html&gt;"); } } </code></pre> <p>for this to work with tomcat and eclipse you may refer to <a href="https://stackoverflow.com/questions/6697063/adding-javaagent-to-tomcat-6-server-where-do-i-put-it-and-in-what-format">Adding -javaagent to Tomcat 6 server, where do I put it and in what format?</a> and <a href="https://stackoverflow.com/questions/13870225/how-to-set-jvm-arguments-in-tomcat-that-work-both-in-eclipse-and-using-the-start">How to set JVM arguments in tomcat that work both in eclipse and using the startup.bat</a></p>
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