How to make a Java thread wait for another thread's output?

Use a CountDownLatch with a counter of 1.

CountDownLatch latch = new CountDownLatch(1);

Now in the app thread do-

latch.await();

In the db thread, after you are done, do -

latch.countDown();

I would really recommend that you go through a tutorial like Sun's Java Concurrency before you commence in the magical world of multithreading.

There are also a number of good books out (google for "Concurrent Programming in Java", "Java Concurrency in Practice".

To get to your answer:

In your code that must wait for the dbThread, you must have something like this:

//do some work
synchronized(objectYouNeedToLockOn){
    while (!dbThread.isReady()){
        objectYouNeedToLockOn.wait();
    }
}
//continue with work after dbThread is ready

In your dbThread's method, you would need to do something like this:

//do db work
synchronized(objectYouNeedToLockOn){
    //set ready flag to true (so isReady returns true)
    ready = true;
    objectYouNeedToLockOn.notifyAll();
}
//end thread run method here

The objectYouNeedToLockOn I'm using in these examples is preferably the object that you need to manipulate concurrently from each thread, or you could create a separate Object for that purpose (I would not recommend making the methods themselves synchronized):

private final Object lock = new Object();
//now use lock in your synchronized blocks

To further your understanding:
There are other (sometimes better) ways to do the above, e.g. with CountdownLatches, etc. Since Java 5 there are a lot of nifty concurrency classes in the java.util.concurrent package and sub-packages. You really need to find material online to get to know concurrency, or get a good book.

Requirement ::

1. To wait execution of next thread until previous finished.
2. Next thread must not start until previous thread stops, irrespective of time consumption.
3. It must be simple and easy to use.

Answer ::

@See java.util.concurrent.Future.get() doc.

future.get() Waits if necessary for the computation to complete, and then retrieves its result.

Job Done!! See example below

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.junit.Test;

public class ThreadTest {

    public void print(String m) {
        System.out.println(m);
    }

    public class One implements Callable<Integer> {

        public Integer call() throws Exception {
            print("One...");
            Thread.sleep(6000);
            print("One!!");
            return 100;
        }
    }

    public class Two implements Callable<String> {

        public String call() throws Exception {
            print("Two...");
            Thread.sleep(1000);
            print("Two!!");
            return "Done";
        }
    }

    public class Three implements Callable<Boolean> {

        public Boolean call() throws Exception {
            print("Three...");
            Thread.sleep(2000);
            print("Three!!");
            return true;
        }
    }

    /**
     * @See java.util.concurrent.Future.get() doc
     *      <p>
     *      Waits if necessary for the computation to complete, and then
     *      retrieves its result.
     */
    @Test
    public void poolRun() throws InterruptedException, ExecutionException {
        int n = 3;
        // Build a fixed number of thread pool
        ExecutorService pool = Executors.newFixedThreadPool(n);
        // Wait until One finishes it's task.
        pool.submit(new One()).get();
        // Wait until Two finishes it's task.
        pool.submit(new Two()).get();
        // Wait until Three finishes it's task.
        pool.submit(new Three()).get();
        pool.shutdown();
    }
}

Output of this program ::

One...
One!!
Two...
Two!!
Three...
Three!!

You can see that takes 6sec before finishing its task which is greater than other thread. So Future.get() waits until the task is done.

If you don't use future.get() it doesn't wait to finish and executes based time consumption.

Good Luck with Java concurrency.