Java------multi-threaded---deadlock

Deadlock:

Concept:

The so-called deadlock: refers to two or more than two Process in the process of execution, because of the contention for the resources of a mutual waiting phenomenon, if there is no external force, they will not be able to proceed. At this point the system is in a deadlock state or the system generates a deadlock, and these processes, which are always waiting on each other, are called deadlock processes. Because resource consumption is mutually exclusive, when a process requests resources, so that the process without external assistance, can never allocate the necessary resources and can not continue to run, which creates a special phenomenon: deadlock.

Conditions for deadlocks to occur:

1. Mutex condition: A resource can only be used by one thread at a time

2. Non-preemption conditions (non-deprivation): The current process locks the resources, before the end, cannot forcibly deprive

3. Possession and application conditions (request and hold conditions): a process has a certain amount of resources, but also want to apply for other resources, but not to their own resources to give up

4. Cycle conditions: A relationship between a number of processes forming a tail-to-toe cyclic waiting resource

When does it usually appear? Nested synchronization in synchronization

Create a deadlock:

Class Test2 implements runnable{private Boolean flag;    Test2 (Boolean flag) {This.flag = flag; } @Override public void Run () {//TODO auto-generated method stub if (flag) {sy                Nchronized (Mylock.locka) {System.out.println (Thread.CurrentThread (). GetName () + "if Locka"); Synchronized (mylock.lockb) {System.out.println (Thread.CurrentThread (). GE                Tname () + "if lockb"); }}} else {synchronized (mylock.lockb) {System.out                . println (Thread.CurrentThread (). GetName () + "Else lockb");  Synchronized (Mylock.locka) {System.out.println (Thread.CurrentThread (). GetName () + "Else                Locka ");    }}}}}class mylock{static Object Locka = new Object (); Static Object lockb = new Object ();} public class Dielocktest{public static void Main (string[] args) {Thread T1 = new Thread (new Test2 (true));        Thread t2 = new Thread (new Test2 (false));        T1.start ();    T2.start (); }}

The program that verifies the synchronization function before the lock is this also becomes a deadlock:

Class Test1 implements runnable{private static int num = 500;    Object obj = new Object ();    Boolean flag = true; public void Run () {if (flag) {while (true) {synchronized (obj)/            /Lock is obj {show ();//Lock Is this}}} else {            while (true) {show ();            }}} public synchronized void Show ()//Lock is this {synchronized (obj)//Lock is obj {                if (num >= 0) {try {thread.sleep (20); } catch (Exception e) {//Todo:handle Exception Syst                Em.out.println (E.tostring ());            } System.out.println (Thread.CurrentThread (). GetName () + ">>--code--" +num--); }}}}public Class dielock{publicstatic void Main (string[] args) {Test1 t = new Test1 ();        Thread A = new thread (t);        Thread B = new Thread (t);        A.start ();        try {thread.sleep (20);        } catch (Exception e) {//Todo:handle Exception} T.flag = false;    B.start (); }}

Prevention of deadlocks:

The prevention of deadlock is a strategy to ensure that the system does not enter the deadlock state.

Know the conditions of the deadlock, to avoid the deadlock will start from breaking the conditions.

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Java------multi-threaded---deadlock