PROGRAMMING IN JAVA - LECTURE -56- SYNCHRONIZATION & INTER-THREAD COMMUNICATION - BCA- S3. смотреть онлайн

Synchronization
• Multithreading introduces asynchronous behavior to the programs. If a thread is writing some data another thread may be reading the same data at that time. This may bring inconsistency.
• When two or more threads need access to a shared resource there should be some way that the resource will be used only by one resource at a time. The process to achieve this is called synchronization.
• To implement the synchronous behavior java has synchronous method. Once a thread is inside a synchronized method, no other thread can call any other synchronized method on the same object. All the other threads then wait until the first thread come out of the synchronized block.
• When we want to synchronize access to objects of a class which was not designed for the multithreaded access and the code of the method which needs to be accessed synchronously is not available with us, in this case we cannot add the synchronized to the appropriate methods. In java we have the solution for this, put the calls to the methods (which needs to be synchronized) defined by this class inside a synchronized block in following manner.
• General syntax.
• synchronized method name{//synchronized method
• ……body of the method;
• }
• Eg:
• synchronized void printTable(int n){//synchronized method
• for(int i=1;i=5;i++){
• System.out.println(n*i);
• }
Inter-thread Communication
We have few methods through which java threads can communicate with each other. These methods are wait(), notify(), notifyAll(). All these methods can only be called from within a synchronized method.
1) To understand synchronization java has a concept of monitor. Monitor can be thought of as a box which can hold only one thread. Once a thread enters the monitor all the other threads have to wait until that thread exits the monitor.
final void notify()
2) wait() tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and calls notify().
final void wait()
3) notify() wakes up the first thread that called wait() on the same object.
notifyAll() wakes up all the threads that called wait() on the same object. The highest priority thread will run first.
final void notifyall()