Master Java Debugging with Arthas: Solve 8 Real‑World Problems

Introduction

Arthas

is Alibaba's open‑source Java diagnostic tool. It can help solve common problems such as locating the JAR a class is loaded from, checking why code changes are not taking effect, debugging without logs, monitoring JVM status, generating flame graphs, and finding class instances.

Problem List

Which JAR loads a class? Why class‑related exceptions?

Why code changes are not taking effect?

How to debug online without redeploying?

How to handle data issues that cannot be reproduced locally?

Is there a global view of system health?

How to monitor JVM real‑time status?

How to locate hotspots and generate flame graphs?

How to find instances of a class inside the JVM?

Solution Overview

The author provides detailed solutions using Arthas commands.

Preparation

Test Code

package com.shockang.study;

import com.alibaba.fastjson.JSON;
import lombok.*;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class ArthasDemo {
    public static void main(String[] args) {
        String s = "[{\"name\":\"zhangsan\",\"age\":\"10\",\"telephone\":\"123456\",\"interests\":[\"sing\",\"dance\",\"rap\"]},
" +
                "{\"name\":\"lisi\",\"age\":\"20\",\"telephone\":\"123457\",\"interests\":[\"sing\",\"swim\"]},
" +
                "{\"name\":\"wangwu\",\"age\":\"30\",\"telephone\":\"123458\",\"interests\":[\"sing\",\"program\"]}]";
        while (true) {
            System.out.println(new ArthasDemo().convert(s));
            try {
                TimeUnit.SECONDS.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private List<People> convert(String s) {
        return JSON.parseArray(s, People.class);
    }

    @Getter @Setter @ToString @FieldDefaults(level = AccessLevel.PRIVATE)
    private static class People {
        String name;
        String age;
        String telephone;
        List<String> interests;
    }
}

Console Output

/Library/Java/JavaVirtualMachines/jdk1.8.0_192.jdk/Contents/Home/bin/java ...
[ArthasDemo.People(name=zhangsan, age=10, telephone=123456, interests=[sing, dance, rap]), ArthasDemo.People(name=lisi, age=20, telephone=123457, interests=[sing, swim]), ArthasDemo.People(name=wangwu, age=30, telephone=123458, interests=[sing, program])]

Download and Run Arthas

Attach to a Java process and open the WebConsole at http://127.0.0.1:3658/.

Problem 1: Locate Class Jar

Use sc -d to find the classloader hash, then

classloader -c <hash> -r com/shockang/study/ArthasDemo.class

to see the JAR path.

sc -d *ArthasDemo*
classloader -c 18b4aac2 -r com/shockang/study/ArthasDemo.class
file:/Users/.../target/classes/com/shockang/study/ArthasDemo.class

Problem 2: Method Not Executed

Use watch and tt to observe method calls, e.g.:

watch com.shockang.study.ArthasDemo convert "{params,target,returnObj}" -f -x 4
tt -t com.shockang.study.ArthasDemo convert

Problem 3: Online Debug Without Redeploy

Use redefine to hot‑replace method bodies, then observe the new behavior without restarting.

Problem 4: Unreproducible Data Issue

Combine tt with tee to dump command output to a file for later analysis.

tt -t com.shockang.study.ArthasDemo convert | tee /Users/shockang/Downloads/log

Problem 5: Global System View

Run dashboard to see real‑time metrics such as QPS, latency, thread‑pool status, etc.

Problem 6: JVM Real‑Time Status

Use jvm command to view CPU, memory, GC, and thread information.

Problem 7: Generate Flame Graph

Run profiler (default CPU) and open the generated files under http://localhost:3658/arthas-output/.

Problem 8: Find Class Instances

Use

vmtool --action getInstances --className java.lang.String --limit 10

to list instances, optionally specifying classloader hash.

vmtool --action getInstances --className java.lang.String --limit 10
@String[][ @String[com/taobao/arthas/core/shell/session/Session], ... ]