Java執行緒之如何分析死鎖及避免死鎖
什麼是死鎖
java中的死鎖是一種程式設計情況,其中兩個或多個執行緒被永久阻塞,Java死鎖情況出現至少兩個執行緒和兩個或更多資源。
在這裡,我們將寫了一個簡單的程式,它將導致java死鎖場景,然後我們將分析它。
怎麼實現死鎖
下面我們一起看一個簡單的死鎖事例,在主執行緒中每隔5秒鐘啟動一個執行緒,每個執行緒的執行時間是3秒鐘
package com.lkf.mulithread; public class ThreadDeadlock { public static void main(String[] args) throws InterruptedException { Object obj1 = new Object(); Object obj2 = new Object(); Object obj3 = new Object(); Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1"); Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2"); Thread t3 = new Thread(new SyncThread(obj3, obj1), "t3"); t1.start(); Thread.sleep(5000); t2.start(); Thread.sleep(5000); t3.start(); } } class SyncThread implements Runnable{ private Object obj1; private Object obj2; public SyncThread(Object o1, Object o2){ this.obj1=o1; this.obj2=o2; } @Override public void run() { String name = Thread.currentThread().getName(); System.out.println(name + " acquiring lock on "+obj1); synchronized (obj1) { System.out.println(name + " acquired lock on "+obj1); work(); System.out.println(name + " acquiring lock on "+obj2); synchronized (obj2) { System.out.println(name + " acquired lock on "+obj2); work(); } System.out.println(name + " released lock on "+obj2); } System.out.println(name + " released lock on "+obj1); System.out.println(name + " finished execution."); } private void work() { try { Thread.sleep(30000); } catch (InterruptedException e) { e.printStackTrace(); } } }
執行結果:
t1 acquiring lock on [email protected]
t1 acquired lock on [email protected]
t2 acquiring lock on [email protected]
t2 acquired lock on [email protected]
t3 acquiring lock on [email protected]
t3 acquired lock on [email protected]
t1 acquiring lock on [email protected]
t2 acquiring lock on [email protected]
t3 acquiring lock on [email protected]
可以看到每個執行緒中都能成功獲取第一個物件的鎖,最終卻阻塞在了獲取第二個物件,造成了執行緒之間的互相等待,形成了死鎖
怎麼排查死鎖
我們通過jstack拿到該應用程式死鎖的執行緒轉儲檔案如下(獲取執行緒轉儲檔案方法):
2018-03-31 15:33:34 Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.161-b12 mixed mode): "RMI TCP Connection(2)-192.168.1.101" #22 daemon prio=9 os_prio=31 tid=0x00007fe2c81cc000 nid=0xa503 runnable [0x000070000ba7c000] java.lang.Thread.State: RUNNABLE at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.socketRead(SocketInputStream.java:116) at java.net.SocketInputStream.read(SocketInputStream.java:171) at java.net.SocketInputStream.read(SocketInputStream.java:141) at java.io.BufferedInputStream.fill(BufferedInputStream.java:246) at java.io.BufferedInputStream.read(BufferedInputStream.java:265) - locked <0x0000000797612b30> (a java.io.BufferedInputStream) at java.io.FilterInputStream.read(FilterInputStream.java:83) at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:550) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$6/556824457.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - <0x0000000797612c78> (a java.util.concurrent.ThreadPoolExecutor$Worker) "JMX server connection timeout 21" #21 daemon prio=9 os_prio=31 tid=0x00007fe2c8968800 nid=0xa703 in Object.wait() [0x000070000b97a000] java.lang.Thread.State: TIMED_WAITING (on object monitor) at java.lang.Object.wait(Native Method) at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:168) - locked <0x0000000797600720> (a [I) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "RMI Scheduler(0)" #20 daemon prio=9 os_prio=31 tid=0x00007fe2c81bc800 nid=0xa903 waiting on condition [0x000070000b877000] java.lang.Thread.State: TIMED_WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x0000000797618010> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject) at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215) at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078) at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1093) at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809) at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "RMI TCP Connection(1)-192.168.1.101" #19 daemon prio=9 os_prio=31 tid=0x00007fe2c884e000 nid=0x5903 runnable [0x000070000b773000] java.lang.Thread.State: RUNNABLE at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.socketRead(SocketInputStream.java:116) at java.net.SocketInputStream.read(SocketInputStream.java:171) at java.net.SocketInputStream.read(SocketInputStream.java:141) at java.io.BufferedInputStream.fill(BufferedInputStream.java:246) at java.io.BufferedInputStream.read(BufferedInputStream.java:265) - locked <0x0000000797600eb8> (a java.io.BufferedInputStream) at java.io.FilterInputStream.read(FilterInputStream.java:83) at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:550) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$6/556824457.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - <0x0000000797600ff8> (a java.util.concurrent.ThreadPoolExecutor$Worker) "RMI TCP Accept-0" #18 daemon prio=9 os_prio=31 tid=0x00007fe2c8855000 nid=0x5703 runnable [0x000070000b671000] 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 sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:52) at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:400) at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:372) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "DestroyJavaVM" #16 prio=5 os_prio=31 tid=0x00007fe2c90cf800 nid=0x2803 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "t3" #14 prio=5 os_prio=31 tid=0x00007fe2c7865000 nid=0x5503 waiting for monitor entry [0x000070000b56e000] java.lang.Thread.State: BLOCKED (on object monitor) at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x0000000797618188> (a java.lang.Object) - locked <0x0000000797618198> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "Attach Listener" #15 daemon prio=9 os_prio=31 tid=0x00007fe2c884d800 nid=0xb07 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "t2" #13 prio=5 os_prio=31 tid=0x00007fe2c884b800 nid=0x3e03 waiting for monitor entry [0x000070000b368000] java.lang.Thread.State: BLOCKED (on object monitor) at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x0000000797618198> (a java.lang.Object) - locked <0x00000007976181e8> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "t1" #12 prio=5 os_prio=31 tid=0x00007fe2c7885000 nid=0x4103 waiting for monitor entry [0x000070000b265000] java.lang.Thread.State: BLOCKED (on object monitor) at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x00000007976181e8> (a java.lang.Object) - locked <0x0000000797618188> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) Locked ownable synchronizers: - None "Service Thread" #11 daemon prio=9 os_prio=31 tid=0x00007fe2c884c800 nid=0x4503 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "C1 CompilerThread2" #10 daemon prio=9 os_prio=31 tid=0x00007fe2c906c000 nid=0x4603 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "C2 CompilerThread1" #9 daemon prio=9 os_prio=31 tid=0x00007fe2c884a800 nid=0x3c03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "C2 CompilerThread0" #8 daemon prio=9 os_prio=31 tid=0x00007fe2c806c800 nid=0x3a03 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "JDWP Command Reader" #7 daemon prio=10 os_prio=31 tid=0x00007fe2c7811000 nid=0x4903 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "JDWP Event Helper Thread" #6 daemon prio=10 os_prio=31 tid=0x00007fe2c7810000 nid=0x3803 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "JDWP Transport Listener: dt_socket" #5 daemon prio=10 os_prio=31 tid=0x00007fe2c780f800 nid=0x4a07 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "Signal Dispatcher" #4 daemon prio=9 os_prio=31 tid=0x00007fe2c801d000 nid=0x3507 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "Finalizer" #3 daemon prio=8 os_prio=31 tid=0x00007fe2c7820800 nid=0x3003 in Object.wait() [0x000070000a847000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143) - locked <0x0000000797618850> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209) Locked ownable synchronizers: - None "Reference Handler" #2 daemon prio=10 os_prio=31 tid=0x00007fe2c8017000 nid=0x2e03 in Object.wait() [0x000070000a744000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) at java.lang.Object.wait(Object.java:502) at java.lang.ref.Reference.tryHandlePending(Reference.java:191) - locked <0x0000000797618890> (a java.lang.ref.Reference$Lock) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153) Locked ownable synchronizers: - None "VM Thread" os_prio=31 tid=0x00007fe2c8014800 nid=0x5303 runnable "GC task thread#0 (ParallelGC)" os_prio=31 tid=0x00007fe2c8011800 nid=0x1b07 runnable "GC task thread#1 (ParallelGC)" os_prio=31 tid=0x00007fe2c8012000 nid=0x1d03 runnable "GC task thread#2 (ParallelGC)" os_prio=31 tid=0x00007fe2c8012800 nid=0x2a03 runnable "GC task thread#3 (ParallelGC)" os_prio=31 tid=0x00007fe2c9010000 nid=0x5403 runnable "VM Periodic Task Thread" os_prio=31 tid=0x00007fe2c781a000 nid=0x4303 waiting on condition JNI global references: 5038 Found one Java-level deadlock: ============================= "t3": waiting to lock monitor 0x00007fe2c91c7198 (object 0x0000000797618188, a java.lang.Object), which is held by "t1" "t1": waiting to lock monitor 0x00007fe2c9046568 (object 0x00000007976181e8, a java.lang.Object), which is held by "t2" "t2": waiting to lock monitor 0x00007fe2c91c5b98 (object 0x0000000797618198, a java.lang.Object), which is held by "t3" Java stack information for the threads listed above: =================================================== "t3": at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x0000000797618188> (a java.lang.Object) - locked <0x0000000797618198> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) "t1": at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x00000007976181e8> (a java.lang.Object) - locked <0x0000000797618188> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) "t2": at com.lkf.mulithread.SyncThread.run(ThreadDeadlock.java:40) - waiting to lock <0x0000000797618198> (a java.lang.Object) - locked <0x00000007976181e8> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) Found 1 deadlock.
為了分析死鎖,我們需要注意那些被阻塞的執行緒,以及等待鎖定的資源,每個資源有一個惟一的ID,我們可以找到哪個執行緒已經持有物件上的鎖,比如:執行緒t3正在等待獲取物件0x00007fe2c91c7198的鎖,但是該物件已經被執行緒t1鎖定。
怎麼避免死鎖
上邊,我們分析了死鎖的情況,發現了導致死鎖的執行緒,我們需要修改程式碼以避免死鎖的情況,那麼怎麼避免死鎖呢
1、避免巢狀鎖
這是死鎖最常見的原因,如果您已經持有一個資源,請避免鎖定另一個資源。如果只使用一個物件鎖,則幾乎不可能出現死鎖情況,比如以下程式碼對上邊的迴圈巢狀部分進行修改,則避免了死鎖的情況:
public void run() {
String name = Thread.currentThread().getName();
System.out.println(name + " acquiring lock on " + obj1);
synchronized (obj1) {
System.out.println(name + " acquired lock on " + obj1);
work();
}
System.out.println(name + " released lock on " + obj1);
System.out.println(name + " acquiring lock on " + obj2);
synchronized (obj2) {
System.out.println(name + " acquired lock on " + obj2);
work();
}
System.out.println(name + " released lock on " + obj2);
System.out.println(name + " finished execution.");
}
2、只鎖需要的部分
只獲對需要的資源加鎖,例如在上面的程式中,我們鎖定了完整的物件資源,但是如果我們只需要其中一個欄位,那麼我們應該只鎖定那個特定的欄位而不是完整的物件
3、避免無限期等待
如果兩個執行緒使用 thread join 無限期互相等待也會造成死鎖,我們可以設定等待的最大時間來避免這種情況。
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Thread.sleep(long millis),一定是當前執行緒呼叫此方法,當前執行緒進入阻塞,但不釋放物件鎖,millis後執行緒自動甦醒進入可執行狀態。作用:給其它執行緒執行機會的最佳方式。Thread.yield(),一定是當前執行緒呼叫此方法,當前執行緒放棄獲取的cpu時間片,由執行狀態變會可執行
Java執行緒的6種狀態及切換(生命週期)
轉自:https://blog.csdn.net/pange1991/article/details/53860651/ Java中執行緒的狀態分為6種。 1. 初始(NEW):新建立了一個執行緒物件,但還沒有呼叫start()方法。2. 執行(RUNNABLE):Java執行緒中將就緒(r
多執行緒之訊號量Semaphore及原理
一、訊號量(Semaphore) 重入鎖ReentrantLock是獨佔模式,一次都只允許一個執行緒訪問一個資源,而訊號量是共享模式,也就是說可以指定多個執行緒,同時訪問某一個資源。 Semaphore的兩個構造方法: public Semaphore(int pe
Java執行緒之銀行存取款
銀行的存取款可以分為存款和取款: 當去存款的時候會先顯示賬戶資訊,然後將存進去的錢和賬戶原有的錢數相加,返回存款之後賬戶資訊; 當去取款的時候會先顯示賬戶資訊,然後將取錢數和賬戶裡面的錢相對比,如果取
Java執行緒之執行緒池的使用
在Java中進行非同步操作時,執行緒必不可少,但如果頻繁的建立、銷燬一個執行緒,這是很耗效能的,所以執行緒池就應運而生了,Java中主要有newFixedThreadPool、newCachedThreadPool、newSingleThreadExecuter及newSchedule