1. Problem: only one lock exists and all the other threads are blocked from running other synchronized methods.

2. Declare separate lock<i> objects and synchronize code blocks as follows:

    Object lock1 = new Object();
    
    public void method() {
    	synchronized(lock1) {
    		// block of code
    	}
    }
   

1. ExecutorService

   1. Has a managerial thread

2. Executor

    public class RunnerThreadPool implements Runnable {
    	private int id;
    	
    	public RunnerThreadPool(int id) {
    		this.id = id;
    	}
    	
    	public void run() {
    		System.out.println("Starting: " + id);
    		
    		try {
    			Thread.sleep(5000);
    		} catch(InterruptedException ie) {
    			ie.printStackTrace();
    		}
    	}
    	
    	public static void main(String[] args) {
    		ExecutorService service = Executors.newFixedThreadPool(2);
    		
    		for(int i = 0; i < 5; i++) {
    			service.submit(new RunnerThreadPool(i));
    		}
    		
    		service.shutdown();
    		
    		try {
    			service.awaitTermination(1, TimeUnit.DAYS);
    		} catch(InterruptedException ie) {
    			ie.printStackTrace();
    		}
    	}
    }
   

1. Semaphore semaphore = new Semaphore(<number-of-permits>);
   1. Initialize with initial number of permits
2. sem.acquire(); (M)
   1. If there are enough permits, continues but if there are no permits, thein waits for permits.
3. sem.release(); (M)
   1. Releases a permit which increments the number of permits of the semaphore.
4. sem.availablePermits();
   1. Returns the number of permits currently available.

Used to return results from thread executing method.

Syntax:

 ExecutorService executor = Executors.newCachedThreadPool();
 
 Future<Integer> future = executor.submit(new Callable<Integer>() {
 	@Override
 	public Integer call() {
 		// some logic that assigns an integer value to 'var'
 		return var;
 	}
 });
 
 try {
 	System.out.println("Return value: " + future.get());
 } catch(InterruptedException e) {
 	e.printStackTrace();
 } catch(ExecutionException e) {
 	e.printStackTrace();
 }