JVM(六)JVM调优命令

事先启动一个web应用程序,用jps查看其进程id,接着用各种jdk自带命令优化应用

  • jps:查看java进程

    PS C:\Users\liaozijian> jps
    54888 jar
    50772 Jps
    
  • jmap: 查看内存信息,实例个数以及占用内存大小

    #jmap -histo 54888 查看历史生成的实例
    #jmap -histo:live 54888 查看当前存活的实例,执行过程中可能会触发一次full gc
    PS C:\Users\liaozijian> jmap -histo 54888 > d:/log.txt
    

    log.txt部分内容如下

     num     #instances         #bytes  class name
    ----------------------------------------------
       1:          8971       23808216  [I
       2:        104987       14049304  [C
       3:         11164        7893848  [B
       4:         89315        2143560  java.lang.String
       5:         23896        2102848  java.lang.reflect.Method
       6:         43886        1404352  java.util.concurrent.locks.AbstractQueuedSynchronizer$Node
       7:         12000        1325584  java.lang.Class
       8:         39461        1262752  java.util.concurrent.ConcurrentHashMap$Node
       9:         27082        1083280  java.util.LinkedHashMap$Entry
      10:         11795         894544  [Ljava.util.HashMap$Node;
      11:         15800         863136  [Ljava.lang.Object;
      12:         13029         729624  java.util.LinkedHashMap
    	..................
    
    • num:序号
    • instances:实例数量
    • bytes:占用空间大小
    • class name:类名称,[C is a char[],[S is a short[],[I is a int[],[B is a byte[],[[I is a int[]

    查看堆信息

    PS C:\Users\liaozijian> jmap -heap 54888
    Attaching to process ID 54888, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 25.161-b12
    
    using thread-local object allocation.
    Parallel GC with 4 thread(s)
    
    Heap Configuration:
       MinHeapFreeRatio         = 0
       MaxHeapFreeRatio         = 100
       MaxHeapSize              = 4292870144 (4094.0MB)
       NewSize                  = 89128960 (85.0MB)
       MaxNewSize               = 1430781952 (1364.5MB)
       OldSize                  = 179306496 (171.0MB)
       NewRatio                 = 2
       SurvivorRatio            = 8
       MetaspaceSize            = 21807104 (20.796875MB)
       CompressedClassSpaceSize = 1073741824 (1024.0MB)
       MaxMetaspaceSize         = 17592186044415 MB
       G1HeapRegionSize         = 0 (0.0MB)
    
    Heap Usage:
    PS Young Generation
    Eden Space:
       capacity = 321912832 (307.0MB)
       used     = 59181336 (56.439720153808594MB)
       free     = 262731496 (250.5602798461914MB)
       18.384273665735698% used
    From Space:
       capacity = 17825792 (17.0MB)
       used     = 17534448 (16.722152709960938MB)
       free     = 291344 (0.2778472900390625MB)
       98.36560417624081% used
    To Space:
       capacity = 19922944 (19.0MB)
       used     = 0 (0.0MB)
       free     = 19922944 (19.0MB)
       0.0% used
    PS Old Generation
       capacity = 132644864 (126.5MB)
       used     = 18898856 (18.023353576660156MB)
       free     = 113746008 (108.47664642333984MB)
       14.247710337280756% used
    
    27696 interned Strings occupying 2852544 bytes.
    

    堆内存 dump

    jmap -dump:format=b,file=eureka.hprof 54888
    

    也可以设置内存溢出自动导出dump文件

    示例代码

    /**
     * 测试OOM
     * -Xms10M -Xmx10M -XX:+PrintGCDetails -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=D:\jvm.dump
     * @author Zijian Liao
     * @since 1.0
     */
    public class OOMTest {
        static List<Object> list = new ArrayList<>();
    
        public static void main(String[] args) {
            while (true) {
                list.add(new User());
            }
        }
    
        static class User {
            byte[] bytes = new byte[1024];
        }
    }
    

    可以用jvisualvm命令工具导入该dump文件分析

  • jstack:查看线程情况:如死锁,某个线程导致cpu飙高

    示例代码

    public class DeadLockTest {
    
       private static Object lock1 = new Object();
       private static Object lock2 = new Object();
    
       public static void main(String[] args) {
          new Thread(() -> {
             synchronized (lock1) {
                try {
                   System.out.println("thread1 begin");
                   Thread.sleep(5000);
                } catch (InterruptedException e) {
                }
                synchronized (lock2) {
                   System.out.println("thread1 end");
                }
             }
          }).start();
    
          new Thread(() -> {
             synchronized (lock2) {
                try {
                   System.out.println("thread2 begin");
                   Thread.sleep(5000);
                } catch (InterruptedException e) {
                }
                synchronized (lock1) {
                   System.out.println("thread2 end");
                }
             }
          }).start();
    
          System.out.println("main thread end");
       }
    }
    
    PS C:\Users\liaozijian> jstack 53460
    Found one Java-level deadlock:
    =============================
    "Thread-1":
      waiting to lock monitor 0x000000001c451288 (object 0x000000076b02c848, a java.lang.Object),
      which is held by "Thread-0"
    "Thread-0":
      waiting to lock monitor 0x000000001c44d1e8 (object 0x000000076b02c858, a java.lang.Object),
      which is held by "Thread-1"
    
    Java stack information for the threads listed above:
    ===================================================
    "Thread-1":
            at com.my.jvm.test.gc.DeadLockTest.lambda$main$1(DeadLockTest.java:30)
            - waiting to lock <0x000000076b02c848> (a java.lang.Object)
            - locked <0x000000076b02c858> (a java.lang.Object)
            at com.my.jvm.test.gc.DeadLockTest$$Lambda$2/1688376486.run(Unknown Source)
            at java.lang.Thread.run(Thread.java:748)
    "Thread-0":
            at com.my.jvm.test.gc.DeadLockTest.lambda$main$0(DeadLockTest.java:17)
            - waiting to lock <0x000000076b02c858> (a java.lang.Object)
            - locked <0x000000076b02c848> (a java.lang.Object)
            at com.my.jvm.test.gc.DeadLockTest$$Lambda$1/38997010.run(Unknown Source)
            at java.lang.Thread.run(Thread.java:748)
    
    Found 1 deadlock.
    
  • jinfo -flags pid:查看jvm参数

  • jinfo -sysprops pid:查看系统参数

  • jstat -gc pid [time] [count] :评估程序内存使用及GC压力整体情况

    • S0C:第一个幸存区的大小,单位KB
    • S1C:第二个幸存区的大小
    • S0U:第一个幸存区的使用大小
    • S1U:第二个幸存区的使用大小
    • EC:伊甸园区的大小
    • EU:伊甸园区的使用大小
    • OC:老年代大小
    • OU:老年代使用大小
    • MC:方法区大小(元空间)
    • MU:方法区使用大小
    • CCSC:压缩类空间大小
    • CCSU:压缩类空间使用大小
    • YGC:年轻代垃圾回收次数
    • YGCT:年轻代垃圾回收消耗时间,单位s
    • FGC:老年代垃圾回收次数
    • FGCT:老年代垃圾回收消耗时间,单位s
    • GCT:垃圾回收消耗总时间,单位s
# JVM 

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