java性能优化常用工具jmap、jstack

jmap：java内存映像工具

jmap用于生成堆转储快照，比较常用的option包括-heap，-histo，-dump

[root@localhost script]# jmap -h
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable <core>
        (to connect to a core file)
    jmap [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    <none>               to print same info as Solaris pmap
    -heap                to print java heap summary
    -histo[:live]        to print histogram of java object heap; if the "live"
                         suboption is specified, only count live objects
    -clstats             to print class loader statistics
    -finalizerinfo       to print information on objects awaiting finalization
    -dump:<dump-options> to dump java heap in hprof binary format
                         dump-options:
                           live         dump only live objects; if not specified,
                                        all objects in the heap are dumped.
                           format=b     binary format
                           file=<file>  dump heap to <file>
                         Example: jmap -dump:live,format=b,file=heap.bin <pid>
    -F                   force. Use with -dump:<dump-options> <pid> or -histo
                         to force a heap dump or histogram when <pid> does not
                         respond. The "live" suboption is not supported
                         in this mode.
    -h | -help           to print this help message
    -J<flag>             to pass <flag> directly to the runtime system

java -heap [vmid]，可以打印制定java虚拟机进程id的堆基本信息及使用情况，包括新生代大小、老年代大小、使用空间大小及比例。

jmap -histo [vmid]，可以打印矩阵图，可以看到java对象实例数量、大小、及具体是哪个对象。

jmap -dump:live,format=b,file=heap.bin <vmid>，可以生成一个heap.bin文件，从服务器上拿下来通过MAT工具进行分析，找到占用内存较高的对象，可以定位内存溢出的错误。

总体来说jmap命令操作简单，但是用来优化系统和排查问题最常用的命令工具。

jstack：线程堆栈信息查看工具

jstack是jdk自带的线程堆栈分析工具,使用该命令可以查看或导出 java 应用程序中线程堆栈信息。

[root@localhost script]# jstack -h
Usage:
    jstack [-l] <pid>
        (to connect to running process)
    jstack -F [-m] [-l] <pid>
        (to connect to a hung process)
    jstack [-m] [-l] <executable> <core>
        (to connect to a core file)
    jstack [-m] [-l] [server_id@]<remote server IP or hostname>
        (to connect to a remote debug server)

Options:
    -F  to force a thread dump. Use when jstack <pid> does not respond (process is hung)
    -m  to print both java and native frames (mixed mode)
    -l  long listing. Prints additional information about locks
    -h or -help to print this help message

jstack可以用来排查CPU使用高、内存使用高的情况，最终定位到问题点。

通过top命令查看CPU（内存）占用高的进程ID，然后通过命令top -H -p <vmid>，定位到具体的线程号pid。

拿到pid后，转为16进制，通过命令printf "%x\n" pid，拿到hexpid

然后通过jstack <vmid> | grep <hexpid> -A 30，最终定位到问题点。

jstack的使用例子：

我首先创建了一个很简单的springboot的项目，然后提供了一个接口，接口的方法体中有个双层循环（方法代码见下面的mockdump），把该项目部署到linux服务器上，启动后的进程id是73165，执行shell命令top -H -p 73165，然后通过浏览器访问该方法。之后观察到线程号73183的cpu使用率高达95%（见下图），然后通过jstack查看线程堆栈信息（见最后的shell命令执行的结果），发现异常信息指向了64行双层循环处，即定位到了代码异常点。

@GetMapping("/mockdump")
    public String mockdump(String loopcount) throws InterruptedException, ExecutionException {
        log.info("===mockdump===start===");
        for (int i = 0; i < Integer.parseInt(loopcount); i++) {
            ExecutorService executor =
                    new ThreadPoolExecutor(2, 10, 30, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
            Future<String> f = executor.submit(new Callable<String>() {
                @Override
                public String call() throws Exception {
                    log.info("===wolaile===");
                    return "wolaile";
                }
            });
            long result = 0l;
            for (int m = 0; m < 999999; m++) {//这个是当前类的第64行
                for (int j = 0; j < 999999999; j++) {
                    result = result + j;
                }
            }
            calllist.add(f.get());
        }
        log.info("===mockdump===end===");
        return "success";
    }

[root@localhost ~]# printf "%x\n" 73183
11ddf
[root@localhost ~]# jstack 73165 | grep 11ddf -A 30
"http-nio-8080-exec-1" #13 daemon prio=5 os_prio=0 tid=0x00007fb33cdd6000 nid=0x11ddf runnable [0x00007fb30e6e7000]
   java.lang.Thread.State: RUNNABLE
        at com.demo.incubator.swaggerdemo.controller.SwaggerDemoController.mockdump(SwaggerDemoController.java:64)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:197)
        at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141)
        at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894)
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)
        at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1063)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
        at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:655)
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:228)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:163)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:190)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:163)
        at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:190)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:163)
        at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)