獲取對象的大小

我想獲取對象的大小。我試圖用這個方法：獲取對象的大小

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); 
    } 
}

但它拋出這個錯誤：

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但我無法找到該對象活着......

我能做什麼？

來源

2013-12-08 Andrea Catania

的premain當一個代理從Java連接被稱爲命令行，例如：'-javaagent：agent.jar'，你在做這個嗎？ – alexgirao

@alexgirao不，因爲我使用tomcat，我可以這樣做嗎？ –

afaik，當你使用java代理時，你會得到instrumentation對象，一個java代理用來監視jvm，也許這不是獲得這個對象的大小的方法 – alexgirao

爲了得到物體的大小使用儀表，有必要的試劑加載到JVM，這裏是代理代碼和貨單

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; 
    } 
}

代理上述允許您這

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可參考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

來源

2013-12-08 12:23:30 alexgirao

請參考documentation。摘錄：

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代理不能促成一個已經運行的JVM;該premain方法在main方法之前調用，再次作爲明確記載：

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.

來源

2013-12-08 09:05:02

我的清單是這樣的：Manifest-Version：1.0 類路徑： Premain-Class：ObjectSizeFetcher但它不工作..我使用tomcat和我是一個web應用程序 –

這是行不通的。代理JAR只能用於主入口點，不能用於部署到已運行的應用程序容器中的Web應用程序。另外，你真的不應該把你的課程放到默認包中。 –

hmmm，所以問題是我如何獲得webapp對象的大小？ –

獲取對象的大小

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