Java Thread Fundamentals

What is a Thread in java?

Thread is a execution path for a java program. A program executes in JVM inside a thread. At least one thread is required to execute a java program. That is called main thread and it is created by JVM itself.

How to create Thread in java?

There are two ways of creating thread in java.

 1. By extending Thread class

    public MyThread extends Thread{

    long priority;

    MyThread (long priority) {

        this.priority= priority;

       }

    public void run() {

    System.out.println("Hello from a thread!");

    }

    }

2. By implementing Runnable interface

    public MyRunnable  implements Runnable{

    long priority;

    MyRunnable (long priority) {

        this.priority= priority;

       }

    public void run() {

    System.out.println("Hello from a thread!");

    }

    }

Extending Thread class or implementing Runnable Interface. Which is better?

You have two ways to do so. First, making your class "extends" Thread class. The other way is making your class implement "Runnable" interface. The latter is more advantageous, cause when you are going for multiple inheritance, then only interface can help. Also, if you are implementing interface, it means you have to implement all methods in the interface. Both Thread class and Runnable interface are provided for convenience and use them as per the requirement.

What invokes a thread's run() method?

After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

How to create Thread objects?

If you extends Thread class , lets say class name is MyThread, then create thread object as below:

MyThread thread1 = new MyThread(1);

thread1 .start();

If you extends Runnable interface , lets say class name is MyRunnable, then create thread object as below:

MyRunnable thread1 = new MyRunnable(1);

new Thread(thread1).start();

What are different ways in which a thread can enter the waiting state?

A thread can enter the waiting state by the following ways:

1. Invoking its sleep() method,

2. By blocking on I/O

3. By unsuccessfully attempting to acquire an object's lock

4. By invoking an object's wait() method.

5. It can also enter the waiting state by invoking its (deprecated) suspend() method.

What do you mean by thread sleep?

Thread.sleep causes the current thread to suspend execution for a specified period. This is an means of making processor time available to the other threads of an application or other applications that running on same JVM.

         Two overloaded versions of sleep are provided: one that specifies the sleep time to the millisecond and one that specifies the sleep time to the nanosecond. However, these sleep times are not guaranteed to be precise.

What is the difference between yielding and sleeping?

When a task invokes its yield() method, it returns to the ready state, either from waiting, running or after its creation. When a task invokes its sleep() method, it returns to the waiting state from a running state.

What is thread join?

The join method allows one thread to wait for the completion of another..

What are synchronized methods and synchronized blocks?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized block can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

What is mutual exclusion? How can you take care of mutual exclusion using Java threads?

Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.

What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

What is the purpose of the wait(), notify(), and notifyAll() methods?

The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication.

 Wait() : waits the calling thread still notify() or notifyAll() is called by another thread

What are the states associated in the thread?

A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.

What is deadlock?

Deadlock is a situation when one thread is waiting for one resource and holding another resource which is to be acquired by another thread and already acquired one resource for which the other thread is waiting.

Example : Thread1 is acquired Resource1 and waiting for Resource2

                  Thead2 is acquired Resource2 but waiting for Resource1

So both the threads will go to a indefinite waiting which will never be broken is called deadlock.

How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

What happens when a thread cannot acquire a lock on an object?

Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it. If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

What’s the difference between the methods sleep() and wait()?

The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.

What is the difference between process and thread?

A thread is a separate path of execution in a program. A Process is a program in execution.

What is daemon thread and which method is used to create the daemon thread?

Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()

When a standalone application is run then as long as any user threads are active the JVM cannot terminate, otherwise the JVM terminates along with any daemon threads which might be active. Thus a daemon thread is at the mercy of the runtime system. Daemon threads exist only to serve user threads.

What is synchronization and why is it important?

With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.

E.g. synchronizing a function:

public synchronized void Method1 () {

// method code.

}

E.g. synchronizing a block of code inside a function:

public Method2 (){

synchronized (this) {

// synchronized code here.

}

}

When you will synchronize a piece of your code?

When you expect that your shared code will be accessed by different threads and these threads may change a particular data causing data corruption, then they are placed in a synchronized construct or a synchronized method.

Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

What is an object's lock and which objects have locks?

 An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

Can a lock be acquired on a class?

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

What state does a thread enter when it terminates its processing?

When a thread terminates its processing, it enters the dead state.

How would you implement a thread pool?

public class ThreadPool implements ThreadPoolInt

This class is an generic implementation of a thread pool, which takes the following input

a) Size of the pool to be constructed

b) Name of the class which implements Runnable and constructs a thread pool with active threads that are waiting for activation. Once the threads have finished processing they come back and wait once again in the pool.

This thread pool engine can be locked i.e. if some internal operation is performed on the pool then it is preferable that the thread engine be locked. Locking ensures that no new threads are issued by the engine. However, the currently executing threads are allowed to continue till they come back to the passivePool.

Is there a separate stack for each thread in Java?

Yes. Every thread maintains its own separate stack, called Runtime Stack but they share the same memory. Elements of the stack are the method invocations,

called activation records or stack frame. The activation record contains pertinent information about a method like local variables.

What is the volatile modifier for?

Volatile variable is used for inter thread communication. It guarantees that if one thread modifies the volatile variable it should be communicated to all other threads.

Why wait() method defined in Object class?

The wait() method is defined in the Object class, which is the ultimate super class of all others. So the Thread class and any Runnable implementation inherit this method from Object. The wait() method is normally called on an object in a multi-threaded program to allow other threads to run. The method should only be called by a thread that has ownership of the object's monitor, which usually means it is in a synchronized method or statement block.

What is a green thread?

A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads.

There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.

What is a working thread?

A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them.

The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.