Java线程在操作系统层面的一些'蛛丝马迹'

书唐瑞

发布于 2022-06-02 14:30:22

1840

发布于 2022-06-02 14:30:22

文章被收录于专栏：Netty历险记

作为Java开发人员,在日常的开发工作中,无时无刻不在和线程打交道.本篇文章并不是讲解线程的相关知识.而是在Linux平台使用一些命令工具,观察下Java线程在操作系统层面的一些'蛛丝马迹'.

实验平台

Win10平台上安装的Ubuntu 18.04 LTS

首先从C语言层面,观察下会有几个线程或进程.代码如下

// thread.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[])
{

    // 为了在使用命令观察程序的时候不让它退出,此处让程序睡眠
    sleep(1000);

    return 0;
}

编译并运行

接下来使用命令查看

ps -ef

进程号=367

再查看下这个进程有几个线程

ps -Lf 367

进程ID等于线程ID, 可以理解为进程里面有且只有一个线程, 这个线程就是进程本身.

接下来,在上面代码的基础上,手动创建一个线程,再观察下会有几个线程.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/syscall.h>

// 子线程执行的任务
void *task(void *arg)
{
    // 线程ID
    pid_t tid = syscall(SYS_gettid);
    printf("child thread, thread ID = %d\n", tid);

    sleep(1000);

}

int main(int argc, char *argv[])
{

    // 打印主线程ID
    pid_t tid = syscall(SYS_gettid);
    printf("main thread, thread ID = %d\n", tid);

    pthread_t thread;
    // 创建子线程
    if(pthread_create(&thread, NULL, task, NULL) != 0) {
        printf("thread creation failed\n");
        exit(1);
    }

    sleep(1000);

    return 0;
}

编译并执行

打印出来的主线程ID=553, 子线程ID=554 查看进程ID

进程ID=553

查看线程

ps -Lf 553

LWP(light weight process)轻量级进程的意思,即线程.

有两个线程,553号线程就是主线程(进程本身),554号线程就是553号线程创建出来的子线程.

接下来使用查看系统调用的方式,验证下线程是如何被创建出来的.

strace -ff -o res.txt ./a.out

打印了两个文件

搜索

grep clone *

线程594使用clone系统调用创建了线程595.

以上只是从C语言角度查看了下线程的相关信息.

那么通过以下Java代码查看下线程的相关信息.

public class ThreadTest {
    private static class MyThread implements Runnable {
        public void run() {
            try {
              System.out.println("child thread");
                Thread.sleep(20000);
            } catch (Exception x) {}
        }
    }


    public static void main(String[] args) throws Exception {

        Thread thread = new Thread(new MyThread(), "my_thread");
        thread.start();    
        System.out.println("main thread");
        Thread.sleep(20000);
    }
}

编译之后,通过

strace -ff -o xxx.txt java ThreadTest

命令查看系统调用

生成如下文件(19个文件)

每个文件对应一个线程.同样是一个简单的程序,Java代码创建的线程的确比上面的C语言创建的线程多.毕竟Java会有一些垃圾回收的线程.

查看进程

进程ID=3224

查看线程

共19个线程,与生成的19个文件一一对应.

执行

git clone *

打印的内容如下

xxx.txt.3224:clone(child_stack=0x7f448e75ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f448e7609d0, tls=0x7f448e760700, child_tidptr=0x7f448e7609d0) = 3225
xxx.txt.3225:clone(child_stack=0x7f448c12ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f448c1309d0, tls=0x7f448c130700, child_tidptr=0x7f448c1309d0) = 3226
xxx.txt.3225:clone(child_stack=0x7f4483ebffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4483ec09d0, tls=0x7f4483ec0700, child_tidptr=0x7f4483ec09d0) = 3227
xxx.txt.3225:clone(child_stack=0x7f4483daffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4483db09d0, tls=0x7f4483db0700, child_tidptr=0x7f4483db09d0) = 3228
xxx.txt.3225:clone(child_stack=0x7f4483c9ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4483ca09d0, tls=0x7f4483ca0700, child_tidptr=0x7f4483ca09d0) = 3229
xxx.txt.3225:clone(child_stack=0x7f4483b8ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4483b909d0, tls=0x7f4483b90700, child_tidptr=0x7f4483b909d0) = 3230
xxx.txt.3225:clone(child_stack=0x7f4483a7ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4483a809d0, tls=0x7f4483a80700, child_tidptr=0x7f4483a809d0) = 3231
xxx.txt.3225:clone(child_stack=0x7f448396ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44839709d0, tls=0x7f4483970700, child_tidptr=0x7f44839709d0) = 3232
xxx.txt.3225:clone(child_stack=0x7f448385ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44838609d0, tls=0x7f4483860700, child_tidptr=0x7f44838609d0) = 3233
xxx.txt.3225:clone(child_stack=0x7f44812affb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44812b09d0, tls=0x7f44812b0700, child_tidptr=0x7f44812b09d0) = 3234
xxx.txt.3225:clone(child_stack=0x7f448119ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44811a09d0, tls=0x7f44811a0700, child_tidptr=0x7f44811a09d0) = 3235
xxx.txt.3225:clone(child_stack=0x7f448108ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44810909d0, tls=0x7f4481090700, child_tidptr=0x7f44810909d0) = 3236
xxx.txt.3225:clone(child_stack=0x7f4480c4ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4480c509d0, tls=0x7f4480c50700, child_tidptr=0x7f4480c509d0) = 3237
xxx.txt.3225:clone(child_stack=0x7f4480b3ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4480b409d0, tls=0x7f4480b40700, child_tidptr=0x7f4480b409d0) = 3238
xxx.txt.3225:clone(child_stack=0x7f4480a2ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f4480a309d0, tls=0x7f4480a30700, child_tidptr=0x7f4480a309d0) = 3239
xxx.txt.3225:clone(child_stack=0x7f448091ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44809209d0, tls=0x7f4480920700, child_tidptr=0x7f44809209d0) = 3240
xxx.txt.3225:clone(child_stack=0x7f448080ffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44808109d0, tls=0x7f4480810700, child_tidptr=0x7f44808109d0) = 3241
xxx.txt.3225:clone(child_stack=0x7f44806fffb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f44807009d0, tls=0x7f4480700700, child_tidptr=0x7f44807009d0) = 3242

线程3224(即进程本身)调用clone系统调用创建出来一个3225这个主线程.主线程3225又把其他17个线程调用clone系统调用也创建出来了.

为什么说3225就是主线程呢?我们在代码中打印了