multithreading - Java - 4 Threads manipulate within two synchronized methods the same object data -

first of new here, hope things right during questioning.

the problem:

i have class store attribute int size;

in class 2 methods manipulate size attribute.

`public synchronized void leave(){    this.size++; }`  `public synchronized void enter(){    while(this.size==0){ }    this.size--; }` 

if initialize store object size=2;for example , 4 other objects (the 4 threads) alternately try leave() or enter()the store object endless loop. thought if write synchronized methods, leave method called thread although other thread hang in endless loop.

i hope question understandable. thank help.

first of new here, hope things right during questioning.

your question asked got @ least right :)

4 other objects (the 4 threads) alternately try leave() or enter()the store object endless loop

when 1 thread enters synchronized block, declaring no other threads can enter synchronized region of same object until thread has left initial synchronized block. have thread invoking enter , spinning until size 0. happen, size needs incremented, cannot happen because thread cannot invoke leave (while other thread spinning ever).

solution

instead of busy spinning ( while(<some condition>){ } ), have thread wait on monitor. give lock thread can enter. , after leaveing, notify waiting threads.

public synchronized void leave(){    this.size++;    this.notify(); }  public synchronized void enter(){    while(this.size==0){         this.wait();    }    this.size--; }