Java虚拟机工具之堆栈跟踪工具jstack检测输入等待

当代码运行到有输入的时候，毫无疑问执行会停下来，然后监听键盘时间，然后将键盘输入的字符放到变量中，jstack可以检测这样的

有输入的代码比较简单：

import java.util.Scanner;

public class InputWait {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
String str = s.next();
System.out.println(str);
}
}

通过jps查找到该进程的pid，然后用jstack打印一下该进程的当前状态：

$ jstack -l 28957
2016-11-07 15:44:05
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.101-b13 mixed mode):

"Attach Listener" #10 daemon prio=9 os_prio=0 tid=0x00007f994c001000 nid=0x7164 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"Monitor Ctrl-Break" #9 daemon prio=5 os_prio=0 tid=0x00007f9984142800 nid=0x713c runnable [0x00007f9960950000]
java.lang.Thread.State: RUNNABLE
at java.net.PlainSocketImpl.socketAccept(Native Method)
at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
at java.net.ServerSocket.implAccept(ServerSocket.java:545)
at java.net.ServerSocket.accept(ServerSocket.java:513)
at com.intellij.rt.execution.application.AppMain$1.run(AppMain.java:90)
at java.lang.Thread.run(Thread.java:745)

Locked ownable synchronizers:
- None

"Service Thread" #8 daemon prio=9 os_prio=0 tid=0x00007f99840c0800 nid=0x7138 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"C1 CompilerThread2" #7 daemon prio=9 os_prio=0 tid=0x00007f99840bd000 nid=0x7136 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"C2 CompilerThread1" #6 daemon prio=9 os_prio=0 tid=0x00007f99840bb800 nid=0x7135 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"C2 CompilerThread0" #5 daemon prio=9 os_prio=0 tid=0x00007f99840b8800 nid=0x7133 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"Signal Dispatcher" #4 daemon prio=9 os_prio=0 tid=0x00007f99840b7000 nid=0x7131 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE

Locked ownable synchronizers:
- None

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f9984084000 nid=0x712e in Object.wait() [0x00007f9961c18000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000ec188ee0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x00000000ec188ee0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

Locked ownable synchronizers:
- None

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f998407f800 nid=0x712d in Object.wait() [0x00007f9961d19000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000ec186b50> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x00000000ec186b50> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

Locked ownable synchronizers:
- None

"main" #1 prio=5 os_prio=0 tid=0x00007f998400a800 nid=0x7121 runnable [0x00007f998c32f000]
java.lang.Thread.State: RUNNABLE
at java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:255)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
- locked <0x00000000ec19c578> (a java.io.BufferedInputStream)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
- locked <0x00000000ec267238> (a java.io.InputStreamReader)
at java.io.InputStreamReader.read(InputStreamReader.java:184)
at java.io.Reader.read(Reader.java:100)
at java.util.Scanner.readInput(Scanner.java:804)
at java.util.Scanner.next(Scanner.java:1369)
at InputWait.main(InputWait.java:9)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Locked ownable synchronizers:
- None

"VM Thread" os_prio=0 tid=0x00007f9984078000 nid=0x712b runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f998401f800 nid=0x7122 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f9984021000 nid=0x7124 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f9984023000 nid=0x7127 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f9984024800 nid=0x7129 runnable

"VM Periodic Task Thread" os_prio=0 tid=0x00007f99840c3000 nid=0x713b waiting on condition

JNI global references: 15

来来来，看这段：

"main" #1 prio=5 os_prio=0 tid=0x00007f998400a800 nid=0x7121 runnable [0x00007f998c32f000]
java.lang.Thread.State: RUNNABLE
at java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:255)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
- locked <0x00000000ec19c578> (a java.io.BufferedInputStream)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
- locked <0x00000000ec267238> (a java.io.InputStreamReader)
at java.io.InputStreamReader.read(InputStreamReader.java:184)
at java.io.Reader.read(Reader.java:100)
at java.util.Scanner.readInput(Scanner.java:804)
at java.util.Scanner.next(Scanner.java:1369)
at InputWait.main(InputWait.java:9)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Locked ownable synchronizers:
- None

这里就是告诉我们现在进程正在等待输入。