本文中我将展示一段垃圾代码，这段代码会产生死锁，这样围绕这段代码重点展示三种不同的方法来分析线程日志，从而得知什么地方有问题。

下面的讨论将用到两个类 Account 和 DeadlockDemo classes 定义如下：

public class Account {

private final int number;

private int balance;

public Account(int number, int openingBalance) {
    this.number = number;
    this.balance = openingBalance;
  }

public void withdraw(int amount) throws OverdrawnException {

if (amount > balance) {
      throw new OverdrawnException();
    }

balance -= amount;
  }

public void deposit(int amount) {

balance += amount;
  }

public int getNumber() {
    return number;
  }

public int getBalance() {
    return balance;
  }
}

The above class models a bank account with attributes of account number and balance, and operations such as
deposit(...) and
withdraw(...).
withdraw(...) will throw a simple checked exception,
OverdrawnException, if the amount to withdraw is greater than the available balance.

The remaining classes in the example code are
DeadlockDemo and its nested class
BadTransferOperation.

public class DeadlockDemo {

private static final int NUM_ACCOUNTS = 10;
  private static final int NUM_THREADS = 20;
  private static final int NUM_ITERATIONS = 100000;
  private static final int MAX_COLUMNS = 60;

static final Random rnd = new Random();

List<Account> accounts = new ArrayList<Account>();

public static void main(String args[]) {

DeadlockDemo demo = new DeadlockDemo();
    demo.setUp();
    demo.run();
  }

void setUp() {

for (int i = 0; i < NUM_ACCOUNTS; i++) {
      Account account = new Account(i, rnd.nextInt(1000));
      accounts.add(account);
    }
  }

void run() {

for (int i = 0; i < NUM_THREADS; i++) {
      new BadTransferOperation(i).start();
    }
  }

class BadTransferOperation extends Thread {

int threadNum;

BadTransferOperation(int threadNum) {
      this.threadNum = threadNum;
    }

@Override
    public void run() {

for (int i = 0; i < NUM_ITERATIONS; i++) {

Account toAccount = accounts.get(rnd.nextInt(NUM_ACCOUNTS));
        Account fromAccount = accounts.get(rnd.nextInt(NUM_ACCOUNTS));
        int amount = rnd.nextInt(1000);

if (!toAccount.equals(fromAccount)) {
          try {
            transfer(fromAccount, toAccount, amount);
            System.out.print(".");
          } catch (OverdrawnException e) {
            System.out.print("-");
          }

printNewLine(i);
        }
      }
      // This will never get to here...
      System.out.println("Thread Complete: " + threadNum);
    }

private void printNewLine(int columnNumber) {

if (columnNumber % MAX_COLUMNS == 0) {
        System.out.print("\n");
      }
    }

/**
     * The clue to spotting deadlocks is in the nested locking - synchronized keywords. Note that the locks DON'T
     * have to be next to each other to be nested.
     */
    private void transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {

synchronized (fromAccount) {
        synchronized (toAccount) {
          fromAccount.withdraw(transferAmount);
          toAccount.deposit(transferAmount);
        }
      }
    }
  }
}

首先拿到线程日志当运行程序
DeadlockDemo，日志如下：

2012-10-16 13:37:03

Full thread dump Java HotSpot(TM) 64-Bit Server VM (20.10-b01-428 mixed mode):

"DestroyJavaVM" prio=5 tid=7f9712001000 nid=0x110247000 waiting on condition [00000000]

   java.lang.Thread.State: RUNNABLE

"Thread-21" prio=5 tid=7f9712944000 nid=0x118d76000 waiting for monitor entry [118d75000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366ee0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-20" prio=5 tid=7f971216c000 nid=0x118c73000 waiting for monitor entry [118c72000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e98> (a threads.deadlock.Account)

 - locked <7f3366f58> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-19" prio=5 tid=7f9712943800 nid=0x118b70000 waiting for monitor entry [118b6f000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366f40> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-18" prio=5 tid=7f9712942800 nid=0x118a6d000 waiting for monitor entry [118a6c000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366f40> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-17" prio=5 tid=7f9712942000 nid=0x11896a000 waiting for monitor entry [118969000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366ec8> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-16" prio=5 tid=7f9712941000 nid=0x118867000 waiting for monitor entry [118866000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366ec8> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-15" prio=5 tid=7f9712940800 nid=0x118764000 waiting for monitor entry [118763000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366ef8> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-14" prio=5 tid=7f971293f800 nid=0x118661000 waiting for monitor entry [118660000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366f28> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-13" prio=5 tid=7f97129ae000 nid=0x11855e000 waiting for monitor entry [11855d000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-12" prio=5 tid=7f97129ad000 nid=0x11845b000 waiting for monitor entry [11845a000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366f40> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-11" prio=5 tid=7f97129ac800 nid=0x118358000 waiting for monitor entry [118357000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366eb0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-10" prio=5 tid=7f97129ab800 nid=0x118255000 waiting for monitor entry [118254000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-9" prio=5 tid=7f97129ab000 nid=0x118152000 waiting for monitor entry [118151000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e98> (a threads.deadlock.Account)

 - locked <7f3366ec8> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-8" prio=5 tid=7f97129aa000 nid=0x11804f000 waiting for monitor entry [11804e000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 - locked <7f3366f28> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-7" prio=5 tid=7f97129a9800 nid=0x117f4c000 waiting for monitor entry [117f4b000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 - locked <7f3366e80> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-6" prio=5 tid=7f97129a8800 nid=0x117e49000 waiting for monitor entry [117e48000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366e80> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-5" prio=5 tid=7f97128a1800 nid=0x117d46000 waiting for monitor entry [117d45000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:81)

 - waiting to lock <7f3366f28> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-4" prio=5 tid=7f97121af800 nid=0x117c43000 waiting for monitor entry [117c42000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e80> (a threads.deadlock.Account)

 - locked <7f3366e98> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-3" prio=5 tid=7f97121ae800 nid=0x117b40000 waiting for monitor entry [117b3f000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e80> (a threads.deadlock.Account)

 - locked <7f3366ef8> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-2" prio=5 tid=7f971224a000 nid=0x117a3d000 waiting for monitor entry [117a3c000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 - locked <7f3366f40> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"RMI TCP Accept-0" daemon prio=5 tid=7f97128fd800 nid=0x117837000 runnable [117836000]

   java.lang.Thread.State: RUNNABLE

 at java.net.PlainSocketImpl.socketAccept(Native Method)

 at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:408)

 - locked <7f32ee740> (a java.net.SocksSocketImpl)

 at java.net.ServerSocket.implAccept(ServerSocket.java:462)

 at java.net.ServerSocket.accept(ServerSocket.java:430)

 at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:34)

 at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:369)

 at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:341)

 at java.lang.Thread.run(Thread.java:680)

"Poller SunPKCS11-Darwin" daemon prio=1 tid=7f97128fd000 nid=0x117734000 waiting on condition [117733000]

   java.lang.Thread.State: TIMED_WAITING (sleeping)

 at java.lang.Thread.sleep(Native Method)

 at sun.security.pkcs11.SunPKCS11$TokenPoller.run(SunPKCS11.java:692)

 at java.lang.Thread.run(Thread.java:680)

"Low Memory Detector" daemon prio=5 tid=7f971209e000 nid=0x1173ec000 runnable [00000000]

   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=9 tid=7f971209d000 nid=0x1172e9000 waiting on condition [00000000]

   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=9 tid=7f971209c800 nid=0x1171e6000 waiting on condition [00000000]

   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=9 tid=7f971209b800 nid=0x1170e3000 waiting on condition [00000000]

   java.lang.Thread.State: RUNNABLE

"Surrogate Locker Thread (Concurrent GC)" daemon prio=5 tid=7f971209a800 nid=0x116fe0000 waiting on condition [00000000]

   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=7f971209a000 nid=0x116d1c000 in Object.wait() [116d1b000]

   java.lang.Thread.State: WAITING (on object monitor)

 at java.lang.Object.wait(Native Method)

 - waiting on <7f3001300> (a java.lang.ref.ReferenceQueue$Lock)

 at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)

 - locked <7f3001300> (a java.lang.ref.ReferenceQueue$Lock)

 at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)

 at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=7f9712099000 nid=0x116c19000 in Object.wait() [116c18000]

   java.lang.Thread.State: WAITING (on object monitor)

 at java.lang.Object.wait(Native Method)

 - waiting on <7f30011d8> (a java.lang.ref.Reference$Lock)

 at java.lang.Object.wait(Object.java:485)

 at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)

 - locked <7f30011d8> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=9 tid=7f9712096800 nid=0x116b16000 runnable 

"Gang worker#0 (Parallel GC Threads)" prio=9 tid=7f9712002800 nid=0x1135c7000 runnable 

"Gang worker#1 (Parallel GC Threads)" prio=9 tid=7f9712003000 nid=0x1136ca000 runnable 

"Concurrent Mark-Sweep GC Thread" prio=9 tid=7f971204d800 nid=0x116790000 runnable

"VM Periodic Task Thread" prio=10 tid=7f97122d4000 nid=0x11793a000 waiting on condition 

"Exception Catcher Thread" prio=10 tid=7f9712001800 nid=0x1103ef000 runnable

JNI global references: 1037

Found one Java-level deadlock:

=============================

"Thread-21":

  waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),

  which is held by "Thread-20"

"Thread-20":

  waiting to lock monitor 7f97118bc108 (object 7f3366e98, a threads.deadlock.Account),

  which is held by "Thread-4"

"Thread-4":

  waiting to lock monitor 7f9711834360 (object 7f3366e80, a threads.deadlock.Account),

  which is held by "Thread-7"

"Thread-7":

  waiting to lock monitor 7f97118b9708 (object 7f3366eb0, a threads.deadlock.Account),

  which is held by "Thread-11"

"Thread-11":

  waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),

  which is held by "Thread-20"

Java stack information for the threads listed above:

===================================================

"Thread-21":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366ee0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-20":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e98> (a threads.deadlock.Account)

 - locked <7f3366f58> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-4":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e80> (a threads.deadlock.Account)

 - locked <7f3366e98> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-7":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 - locked <7f3366e80> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-11":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366eb0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

Found 1 deadlock.

Heap

 par new generation   total 19136K, used 11590K [7f3000000, 7f44c0000, 7f44c0000)

  eden space 17024K,  68% used [7f3000000, 7f3b51ac0, 7f40a0000)

  from space 2112K,   0% used [7f40a0000, 7f40a0000, 7f42b0000)

  to   space 2112K,   0% used [7f42b0000, 7f42b0000, 7f44c0000)

 concurrent mark-sweep generation total 63872K, used 0K [7f44c0000, 7f8320000, 7fae00000)

 concurrent-mark-sweep perm gen total 21248K, used 8268K [7fae00000, 7fc2c0000, 800000000)

Scanning quickly through, you can see that this thread dump is divided into four parts. These are:

  1. A complete list of all the applcation’s threads
  2. A list of deadlocked threads
  3. A small stack trace of deadlocked threads
  4. The application’s heap summary

The Thread List

The thread list in point one above is a list of all the application’s threads and their current status. From this you can see that an application consists of a whole bunch of threads, which you can roughly divide in to two. Firstly there are the background threads. These are the ones that every application has, which get on with all the dirty jobs that we, as application programmers, don’t usually need to worry about. These have names such as:
"DestroyJavaVM",
Low Memory Detector,
Finalizer,
Exception Catcher Thread and
Concurrent Mark-Sweep GC Thread. Secondly, there are the threads that you or I may create as part of our code. These usually have names that consist of the word
Thread followed by a number. For example:
Thread-3,
Thread-6 and
Thread-20.

"Thread-20" prio=5 tid=7f971216c000 nid=0x118c73000 waiting for monitor entry [118c72000]

   java.lang.Thread.State: BLOCKED (on object monitor)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:82)

 - waiting to lock <7f3366e98> (a threads.deadlock.Account)

 - locked <7f3366f58> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:58)

Looking at the information given on
Thread-20 in more detail you can see that this can be broken down into several parts. These are:

Thread-20 The thread’s name as described above.
prio=5 The thread's priority. A number from 1 to 10, where 1 is the lowest and 10 is the highest priority.
tid=7f971216c000 The thread id. A unique number that’s returned by a Thread.getId() call.
nid=0x118c73000 The native thread id. This maps to a platform dependent thread id.
waiting for monitor entry [118c72000]
java.lang.Thread.State: BLOCKED (on object monitor) 		This is the status of the thread; in this case it’s BLOCKED. Also included is a stack trace outlining where the thread is blocked.
Note that a thread can also be marked as a
daemon. For example:

"RMI TCP Accept-0" daemon prio=5 tid=7f97128fd800 nid=0x117837000 runnable [117836000]

   java.lang.Thread.State: RUNNABLE

Daemon threads are background task threads such as the RMI TCP Accept-0 thread listed above. A daemon thread is a thread that does not prevent the JVM from exiting. The JVM will exit, or close down, when only daemon threads remain.

However, the thread list doesn't really help in tracing the cause of a deadlock, so moving swiftly along...

The Deadlock Thread List

This section of the thread dump contains a list of all threads that are involved in the deadlock.

Found one Java-level deadlock:

=============================

"Thread-21":

  waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),

  which is held by "Thread-20"

"Thread-20":

  waiting to lock monitor 7f97118bc108 (object 7f3366e98, a threads.deadlock.Account),

  which is held by "Thread-4"

"Thread-4":

  waiting to lock monitor 7f9711834360 (object 7f3366e80, a threads.deadlock.Account),

  which is held by "Thread-7"

"Thread-7":

  waiting to lock monitor 7f97118b9708 (object 7f3366eb0, a threads.deadlock.Account),

  which is held by "Thread-11"

"Thread-11":

  waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),

  which is held by "Thread-20"

From the segment above, you can see that there are five threads all blocking on instances the
threads.deadlock.Account class

Leaving aside the monitor ids and
Account instances, you can see that "Thread-21" is waiting for "Thread-20", which is waiting for "Thread-4", which in turn is waiting for "Thread-7". "Thread-7" is waiting for "Thread-11", which is waiting for "Thread-20": a deadlock loop as shown in the diagram below:

The Deadlock Stack Traces

The final piece of the puzzle is the list of deadlocked thread stack traces as shown below:

Java stack information for the threads listed above:

===================================================

"Thread-21":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366ee0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-20":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e98> (a threads.deadlock.Account)

 - locked <7f3366f58> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-4":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366e80> (a threads.deadlock.Account)

 - locked <7f3366e98> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-7":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366eb0> (a threads.deadlock.Account)

 - locked <7f3366e80> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-11":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f3366f58> (a threads.deadlock.Account)

 - locked <7f3366eb0> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

From the previous section, we know that
Thread-20 is waiting, via a circuitous route, for
Thread-11 and
Thread-11 is waiting for
Thread-20. This is our deadlock.

The next step is to tie this deadlock up to lines of code using the thread stack trace above and I’ve simplified this in the diagram below.

In the above diagram I've removed the
7f3366 prefix from the object ids for clarity; hence, object
7f3366f58 is now
f58. From this diagram, you can see that object
f58 is locked by
Thread-20 on line 59 and is waiting for a lock on object
e98 on line 86. Following the arrows down, you can see that
Thread-7 is waiting for a lock on
eb0 on line 86, which in turn is locked by
Thread-11 on line 59.
Thread-11 is waiting for a lock on
f58 on line 86, which, looping back up, is locked on line 58 by
Thread-20

So, where are these lines of code? The following shows line 59:

...and this is line 86:

Everyone gets surprises sometimes and the stack trace above surprised me. I was expecting the locks to be on lines 85 and 86; however, they were on 59 and 86. Since line 59 doesn’t contain a
synchronized keyword, I’m guessing that the compiler has done some optimisation on the
transfer(...) method’s first
synchronized keyword.

The conclusion that can be drawn from this is that the code, which randomly picks two
Account objects from a list, is locking them in the wrong order on lines 59 and 86. So what’s the fix? More on that next time; however, there’s one final point to note, which is that the make up of a deadlock may not be the same every time you generate a thread dump on a program. After running the
DeadlockDemo program again and using
kill -3 PID to get hold of another thread dump, I obtained these results:

Found one Java-level deadlock:

=============================

"Thread-20":

  waiting to lock monitor 7fdc7c802508 (object 7f311a530, a threads.deadlock.Account),

  which is held by "Thread-3"

"Thread-3":

  waiting to lock monitor 7fdc7a83d008 (object 7f311a518, a threads.deadlock.Account),

  which is held by "Thread-11"

"Thread-11":

  waiting to lock monitor 7fdc7c802508 (object 7f311a530, a threads.deadlock.Account),

  which is held by "Thread-3"

Java stack information for the threads listed above:

===================================================

"Thread-20":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:86)

 - waiting to lock <7f311a530> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-3":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:87)

 - waiting to lock <7f311a518> (a threads.deadlock.Account)

 - locked <7f311a530> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

"Thread-11":

 at threads.deadlock.DeadlockDemo$BadTransferOperation.transfer(DeadlockDemo.java:87)

 - waiting to lock <7f311a530> (a threads.deadlock.Account)

 - locked <7f311a518> (a threads.deadlock.Account)

 at threads.deadlock.DeadlockDemo$BadTransferOperation.run(DeadlockDemo.java:59)

Found 1 deadlock.

In this thread dump a smaller number of threads are involved in the deadlock, but if you analyze it you can draw the same conclusions as my first example.

Next time: fixing the code...

For more information see
the other blogs in this series.

All source code for this an other blogs in the series are available on Github at git://github.com/roghughe/captaindebug.git

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