Question

我想获得一个物体的大小。我试着用这个方法：

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

但它抛出了这个错误：

java.lang.NullPointerException
    test.ObjectSizeFetcher.getObjectSize(ObjectSizeFetcher.java:13)
    servlet.testObj.doGet(cms.java:55)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
    org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:176)
    org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:145)
    org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:92)
    org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:394)

然而我尝试了jprofiler和MAT，但我无法找到这个对象...

我该怎么办？

Answer 1

要使用检测获取对象大小，需要将代理加载到jvm中，这里是代理代码和清单

剂 - MANIFEST.MF

Premain-Class: mypackage.Agent
Agent-Class: mypackage.Agent
Can-Retransform-Classes: true

Agent.java

/* Agent.java

javac -cp ".:$JAVA_HOME/lib/tools.jar" -d . Agent.java Test.java && \
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;
    }
}

上面的代理允许你这个

GetObjectSizeTest.java

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("<html>");
        writer.println("<body bgcolor=white>");
        writer.println("<p>The size of System.in is " + Agent.inst.getObjectSize(System.in) + "</p>");
        writer.println("</body>");
        writer.println("</html>");
    }
}

为了使用tomcat和eclipse你可以参考Adding -javaagent to Tomcat 6 server, where do I put it and in what format?和How to set JVM arguments in tomcat that work both in eclipse and using the startup.bat

Answer 2

请参阅documentation。摘录：

代理JAR文件的清单必须包含属性Premain-Class。此属性的值是代理类的名称。代理类必须实现与主应用程序入口点原则上类似的public static premain方法。

无法将Java代理提供给已经运行的JVM; premain方法在main方法之前被称为，同样明确记录：

在初始化Java虚拟机（JVM）之后，将按指定代理的顺序调用每个premain方法，然后将调用实际应用程序main方法。

获取对象的大小

2 个答案: