Multi-Threading in Java

• A multithreaded program in Java contains two or more parts that can run concurrently, each part of the program is called a thread.
• Each thread defines a separate path of execution, hence we can say that multithreading is a special case of multitasking.

Type of Multitasking

1. Process-based
2. Thread-based

• A process is essentially a program that is executing, hence, process-based multitasking is the feature that allows a computer to run (or execute) two or more program concurrently. For example, Process-based multitasking allows us to run the Java IDE, Zoom, Google Chrome, WhatsApp etc at the same time on our computer. Here, a program is the smallest unit of code that can be dispatched by the scheduler.

• In a thread-based multitasking environment, a thread is the smalled unit of code that can be dispatched by the scheduler. This means that a single program can execute two or more tasks simultaneously. For example, we can still edit the code in our IDE when the compiler is compiling it or the JVM is executing it, as long as these two tasks are being performed on two different threads.

Thus, we can say that,

• The process-based multitasking handles the "big-picture", and
• The thread-based multitasking handles the "fine-details".

Multitasking threads require less resources than multitasking processes.

The Java Thread Model

The multi-threading capabilities of Java are built-in.

States of a thread in Java

1. Running (currently executing)
2. Suspended (paused, after running)
3. Resumed (starting after paused)
4. Blocked (not running, and waiting for a resource)
5. Ready (waiting to run)
6. Terminated (halted OR stopped)

Thread Priorities The Java compiler will assign a priority to a thread. This priority will tell us that how which thread should be treated with respect to others. Basically, these are some integer values. But a higher priority of a thread can not guarantee that a thread will perform its task fast or slow, if it is the only thread running. Priority of a thread can only be observed properly once there are two or more threads running concurrently in a program space. It is basically used to decide that when to switch from one running thread to the next (context switch). There are basic two rules for context switching

1. A thread can voluntarily relinquish(to lose) control: This happens when explicitly yielding, sleeping, or when the thread is blocked. In this, all other threads are examines, and the thread with the highest priority will be marked as ready to run.
2. A thread can be preempted by a higher-priority thread: In this, a thread with a low-priority does not yield the processor, but when it is preempted by a higher-priority processor, it will yield.

The Thread class and the Runnable Interface.

Java's multithreading system is built upon the Thread class, and its companion interface, the Runnable interface.

The Main Thread

• Whenever a Java program executes, one thread starts running immediately, this is called the main thread of the program.

• It is important for two main reasons.

1. It is the thread from which all the other threads will be created.
2. It must be the last thread to finish execution because it performs various shutdown actions.

Creating a Thread

1. By implementing the Runnable interface.
2. By extending the Thread class.