Main thread in Java

When a Java program starts, the Java Virtual Machine (JVM) creates a thread automatically called the main thread. This thread executes the main() method and controls the overall execution flow of the program.

• It is the parent thread from which all other user-defined threads are created.
• The default name of the main thread is "main".
• The default priority of the main thread is 5.
• It usually finishes last as it may perform cleanup and shutdown tasks.

Create the Main Thread

The main thread is created automatically when the program starts. To control it, we must first obtain a reference to it using Thread.currentThread(). This method returns a reference to the currently executing thread.

Current thread: main
After name change: Geeks
Main thread priority: 5
Main thread new priority: 10
Main thread
Main thread
Main thread
Main thread
Main thread
Child thread priority: 10
Child thread new priority: 1
Child thread
Child thread
Child thread
Child thread
Child thread

Explanation:

• Thread.currentThread() returns the main thread.
• getName() and setName() allow reading and changing the thread name.
• getPriority() and setPriority() allow reading and changing the thread priority.
• The main thread can create child threads, which by default inherit the main thread’s priority.

Relationship Between main() Method and Main Thread

• For every Java program, JVM creates the main thread first.
• The main thread checks for the existence of the main() method, then initializes the class.
• Since JDK 6, the main() method is mandatory for a standalone Java application.

Deadlock Using Main Thread

Even a single main thread can cause deadlock if it waits for itself.

Output: 

Explanation:

• Thread.currentThread().join() tells the main thread to wait for itself to die.
• The thread waits indefinitely, causing a deadlock.
• Any code after join() will never execute.