Btrace tells you how to debug online without restarting the JVM

Hello Hello everyone , I'm fan , I don't know if you've ever come across a scene like this , After running online services for a period of time, there will be problems occasionally , Code and data analysis alone cannot find the reason , And this situation is not very common, so the corresponding code does not add log output , If we add logs again to publish , It will destroy the scene. We can only wait for a while , Or sometimes I want to see the parameters of the interface , So as to determine whether there is a problem with the interface parameters .

At this time, I wonder if there is a good way , You can add some logs to see the running status and entry parameters without modifying the source code or publishing the upgrade ？ The answer, of course, is yes , Now let's take a look at the artifact ​​BTrace​​！

Let's simulate a scene , This scenario is that there is a problem with an online service , An error will be reported when some requests are triggered , Now we want to see the details of the input parameters of the method interface when an error is reported .

Write a first ​​demo​​​ Code , An internal loop calls a method , As follows , This code simulates calling a method all the time through a loop , This method ​​print​​​   The input parameter of is a ​​person​​​ Packaging , The internal processing logic of the method is ignored here , And every time 3 Once per second . Now our needs are very simple , I just want to know the parameters of each run ​​person​​​ What is the value of each attribute of , In other words ​​age​​​ and ​​name​​ What's the value of , Of course, we can't modify the source code to add printing and republishing .

Log in and copy

       

        package com.javajeek.demo;
        

        
public class BtraceTest {
        
    public void print(Person person) {
        
        System.out.println("-------- Handle Person ing------");
        
        try {
        
           //... Other logic 
        
            Thread.sleep(3000);
        
        } catch (InterruptedException e) {
        
            e.printStackTrace();
        
        }
        
    }
        

        
    public void test(Person person) {
        
        while (true) {
        
            this.print(person);
        
        }
        
    }
        

        
    public static void main(String[] args) {
        
        BtraceTest btraceTest = new BtraceTest();
        
        Person person = new Person(18, "ziyou");
        
        btraceTest.test(person);
        
    }
        

        
    static class Person {
        
        private int age;
        
        private String name;
        

        
        public Person(int age, String name) {
        
            this.age = age;
        
            this.name = name;
        
        }
        
    }
        
}
       
       

        
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There is nothing to say about the running results , Every time 3 One sentence per second , as follows .

At first glance, everyone may be a little confused , I find it difficult to achieve , This thing is not local ​​debug​​, Can you interrupt . At this time, we need to use our artifact , Although not local breakpoint debugging , But output the attribute value of the parameter and some ​​JVM​​ The state of can still be realized .

BTrace

Before providing a solution , Let's take a look at what ​​BTrace​​​,​​BTrace​​​ yes ​​sun​​​ The company launched a ​​Java​​​ dynamic 、 Safe tracking （ monitor ） Tools , You can do it without restarting ​​JVM​​​ Monitor the operation of the system , Convenient access to the data information when the program is running , Such as method parameters 、 Return value 、 Global variables and stack information . In a nutshell ​​BTrace​​ It's a tool , It can help us get some data of runtime in real time .

Download and install

First, let's download the compressed package ​​wget https://github.com/btraceio/btrace/releases/download/v2.2.0/btrace-v2.2.0-bin.tar.gz​​​ Download the latest version of the compressed package , After downloading, unzip it , Ah fan is here ​​macOS​​​ Shown above , If it is ​​Windows​​​ You can go directly to ​​GitHub​​​ It is the same to download it and decompress it manually .​​GitHub​​​ Address ​​https://github.com/btraceio/btrace/releases/tag/v2.2.0​​.

Log in and copy

       

        wget https://github.com/btraceio/btrace/releases/download/v2.2.0/btrace-v2.2.0-bin.tar.gz
        
tar xzvf btrace-v2.2.0-bin.tar.gz
       
       

        
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After decompression , We can see the directory structure as follows , among ​​samples​​ There is a lot of sample code in the directory. Interested partners can go and have a look ：

Use

In the use of ​​BTrace​​​ We need to write a ​​Java​​​ Script , Express the things we need to deal with in this script , If you want to know ​​JVM​​ How is the operation of , What is the return value of a method of a class , What is the method input parameter and any information you want to know . Here is the script code of the solution in our case , As follows ：

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        import org.openjdk.btrace.core.annotations.*;
        
import org.openjdk.btrace.core.types.*;
        

        
import static org.openjdk.btrace.core.BTraceUtils.*;
        
@BTrace
        
public class AllMethodsTest {
        
    @OnMethod(
        
            clazz = "com.lazada.marketing.search.ads.diamond.BtraceTest",
        
            method = "print",
        
            location = @Location(Kind.ENTRY)
        
    )
        
    public static void m(Object request) {
        
        printFields(request);
        
    }
        
}
       
       

        
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You can see that the core code is just a few lines , Write with us ​​Java​​​ The code is very similar , It is also an import package , Write a class , Redefine the method , At the same time, I added some comments of my own . Through the content of the annotation, we can deal with which class and which class's methods . About ​​BTrace​​ What are the notes , And how each annotation is used. For details, please refer to this article https://ningyu1.github.io/site/post/39-btrace/ Very detailed , Ah fan thinks it's very good , Mention annotations with classes , Comment on method , And comments related to parameters .

Briefly, let's talk about a few comments we used in the above code ,​​@BTrace​​​ ,​​OnMethod​​​,​​@Location​​ The meaning of representation ：

​​@BTrace​​​： Indicates that this class is a ​​BTrace​​​ Program ,​​BTrace​​​ The compiler will force the lookup of this annotation ,​​BTrace​​ The agent will also check whether this annotation exists . without , Then the prompt is wrong , And will not execute .

​​OnMethod​​​： This annotation can be used to specify the target class , The target method , And in the target method “ Location ”. among ​​clazz​​​ and ​​method​​ Respectively represent the class and method to be executed , It can be expressed by regularity .

​​@Location​​： Represents a specific location in the method . For values, please refer to the table in the above article .

perform

With the above running program and ​​BTrace​​​ We can meet our requirements with the sample code of , First of all, our ​​demo​​​ The code is running , We go through ​​jps​​​ The command finds the corresponding ​​pid​​​, The operation is as follows , Corresponding ​​pid​​​ yes ​​84287​​.

We found ​​pid​​ After that, we can execute ​​BTrace​​ Procedure , Through the command ​​./bin/btrace 84287 samples/AllMethodsTest.java​​, The results are shown in the figure below . You can see that the details of the input parameters are output , So far, our requirements have been met . Are the friends very happy , Very excited , Very excited ？ This use is just ​​BTrace​​ Powerful tip of iceberg , There are many better examples in ​​samples​​ Under the table of contents , Interested partners can study .

summary

The above example , A fan is just a very simple but common way to use it , You can study more powerful applications by yourself , Many friends think there must be a doubt when they see this function , That's it ​​BTrace​​​ How to realize this function , A fan shares with you a meituan technology blog post , The introduction is quite detailed and easy to understand , You can go and have a look https://tech.meituan.com/2019/02/28/java-dynamic-trace.html,​​BTrace​​​ Although easy to use , But it is relatively troublesome to use , I believe everyone knows that a better tool is Alibaba open source ​​Arthas​​, You can study .

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