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
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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.txtlog.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
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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