获取对象的大小

我想获取对象的大小。我试图用这个方法：获取对象的大小

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)

但是我tryed的JProfiler和MAT但我无法找到该对象活着......

我能做什么？

来源

2013-12-08 Andrea Catania

的premain当一个代理从Java连接被称为命令行，例如：'-javaagent：agent.jar'，你在做这个吗？ – alexgirao

@alexgirao不，因为我使用tomcat，我可以这样做吗？ –

afaik，当你使用java代理时，你会得到instrumentation对象，一个java代理用来监视jvm，也许这不是获得这个对象的大小的方法 – alexgirao

为了得到物体的大小使用仪表，有必要的试剂加载到JVM，这里是代理代码和货单

代理的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; 
    } 
}

代理上述允许您这

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>"); 
    } 
}

此使用Tomcat工作和eclipse可参考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

来源

2013-12-08 12:23:30 alexgirao

请参考documentation。摘录：

The manifest of the agent JAR file must contain the attribute Premain-Class . The value of this attribute is the name of the agent class. The agent class must implement a public static premain method similar in principle to the main application entry point.

Java代理不能促成一个已经运行的JVM;该premain方法在main方法之前调用，再次作为明确记载：

After the Java Virtual Machine (JVM) has initialized, each premain method will be called in the order the agents were specified, then the real application main method will be called.

来源

2013-12-08 09:05:02

我的清单是这样的：Manifest-Version：1.0 类路径： Premain-Class：ObjectSizeFetcher但它不工作..我使用tomcat和我是一个web应用程序 –

这是行不通的。代理JAR只能用于主入口点，不能用于部署到已运行的应用程序容器中的Web应用程序。另外，你真的不应该把你的课程放到默认包中。 –

hmmm，所以问题是我如何获得webapp对象的大小？ –

获取对象的大小

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