玩耍一下jstack，jinfo，jmap
jstack
jstack用于查看java的某个进程的线程的堆栈信息，先来个死循环。
由于图床有点bug，上传不了图片，所以就先盗别人的图顶着先了。
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9class Jstack {
public static void main(String[] args) {
while (true) {
if (Math.random()==Math.random()) {
System.out.println("zhe me qiao");
}
}
}
}
使用top查看那个java进程占用cpu最多，得到进程pid
然后使用top -Hp 进程pid获得此进程的全部线程
我们就可以发现cpu占用最大的线程的线程pid了，为了接下来好观察，我们需要使用printf "%x\n" 线程pid把线程pid转换为16进制。
最后使用jstack 进程pid，就能查看此进程的线程的堆栈情况。为了找到我们需要的线程，线程的pid在信息里是在nid后面的。但是我发现进行这个死循环一段时间之后就会自动爆出一下信息，然后jstack命令就会查询失败。我用的是jdk8，可能是新jdk的新功能吧。
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3root@cellargalaxy-Lenovo-G50-80:/usr/local/java/jdk1.8.0_131/bin# ./jstack 18287
18287: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding
例如下面这个片段，死循环线程pid是0x4770。
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21"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f0bec083800 nid=0x4777 in Object.wait() [0x00007f0bc74d2000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d6e08ec8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x00000000d6e08ec8> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f0bec07f000 nid=0x4776 in Object.wait() [0x00007f0bc75d3000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000d6e06b68> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x00000000d6e06b68> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
"main" #1 prio=5 os_prio=0 tid=0x00007f0bec009800 nid=0x4770 runnable [0x00007f0bf4082000]
java.lang.Thread.State: RUNNABLE
at Jstack.main(T.java:4)
jinfo
jinfo用来查看jvm的参数，用法是jinfo 进程pid。然后他会罗列好多好多信息。例如执行java Jstack -Xms30m -Xmx60m
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22Attaching to process ID 19664, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
Java System Properties:
java.vm.name = Java HotSpot(TM) 64-Bit Server VM 虚拟机
user.dir = /tmp 执行目录
java.runtime.version = 1.8.0_131-b11 jre版本
os.arch = amd64 cpu？
os.name = Linux 系统
java.class.version = 52.0 class版本
sun.management.compiler = HotSpot 64-Bit Tiered Compilers 还是虚拟机
user.home = 用户目录
user.name = 用户名
sun.java.command = Jstack -Xms30m -Xmx60m 执行java命令时添加的jvm参数
java.version = 1.8.0_131 jdk版本？
VM Flags: 其余乱七八糟的参数？
Non-default VM flags: -XX:CICompilerCount=3 -XX:InitialHeapSize=130023424 -XX:MaxHeapSize=2069889024 -XX:MaxNewSize=689963008 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=42991616 -XX:OldSize=87031808 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseParallelGC
Command line:
jmap
jmap用于查看内存情况，用法：jmap -heap 进程pid
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10import java.util.LinkedList;
public class HeapOOM {
public static void main(String[] args) throws InterruptedException {
LinkedList<HeapOOM> linkedList=new LinkedList<HeapOOM>();
while (true) {
linkedList.add(new HeapOOM());
Thread.sleep(1);
}
}
}
执行java -Xms2m -Xmx2m -XX:+HeapDumpOnOutOfMemoryError HeapOOM，限制一下堆大小。然后好不容易试到了下面的jmap
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46Attaching to process ID 21125, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 2097152 (2.0MB) 指定的最大对内存2m
NewSize = 1572864 (1.5MB)
MaxNewSize = 1572864 (1.5MB)
OldSize = 524288 (0.5MB) 应该是指老年代大小
NewRatio = 2
SurvivorRatio = 8 新生代那个复制算法的比例
MetaspaceSize = 21807104 (20.796875MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 17592186044415 MB
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 524288 (0.5MB) 应该是Eden的大小
used = 335896 (0.32033538818359375MB) Eden用了0.32m
free = 188392 (0.17966461181640625MB) Eden剩余0.179m
64.06707763671875% used Eden用了64%
From Space:
capacity = 524288 (0.5MB)
used = 515640 (0.49175262451171875MB)
free = 8648 (0.00824737548828125MB)
98.35052490234375% used
To Space:
capacity = 524288 (0.5MB)
used = 0 (0.0MB)
free = 524288 (0.5MB)
0.0% used
PS Old Generation 老年代
capacity = 524288 (0.5MB)
used = 385672 (0.36780548095703125MB)
free = 138616 (0.13219451904296875MB)
73.56109619140625% used
654 interned Strings occupying 44000 bytes.
之后的情况
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46Attaching to process ID 21125, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 2097152 (2.0MB)
NewSize = 1572864 (1.5MB)
MaxNewSize = 1572864 (1.5MB)
OldSize = 524288 (0.5MB)
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 = 524288 (0.5MB)
used = 524280 (0.49999237060546875MB) Eden用了0.49m
free = 8 (7.62939453125E-6MB) Eden几乎没剩了
99.99847412109375% used Eden用了99%
From Space:
capacity = 524288 (0.5MB)
used = 515640 (0.49175262451171875MB)
free = 8648 (0.00824737548828125MB)
98.35052490234375% used
To Space:
capacity = 524288 (0.5MB)
used = 0 (0.0MB)
free = 524288 (0.5MB)
0.0% used
PS Old Generation Eden满了，老年代的使用也增加，所剩无几了
capacity = 524288 (0.5MB)
used = 454496 (0.433441162109375MB)
free = 69792 (0.066558837890625MB)
86.688232421875% used
654 interned Strings occupying 44000 bytes.
参考文章：
如何使用jstack分析线程状态