Multithreading: Thread Basics

Thread Basics

• Program contains multiple processes and process consists of a single or multiple threads, thread is also a small lightweight sub-process
• Context switching between threads is less expensive than processes
• Intercommunication between thread is easy than process communication
• There are 2 types of threads - user threads & daemon threads, “main” is first user thread in any java application
• Once thread is started, it’s execution depends on the OS implementation of time slicing, we cannot control/guarantee thread execution
• A thread always inherits properties from parent thread

Thread Creation

• Implement java.lang.Runnable (preferred due to multiple inheritance issue)
• Class will Implement Runnable interface
• Create an object of class
• Create Thread object and pass class reference in #2 to Thread
• Call start() method on Thread object
• Calling start() method internally calls run() method of class
• Extend java.lang.Thread
• Class will extend Thread class
• Create an object of class
• Call start() method on the object
• Calling start() method internally calls run() method of class

 Thread Lifecycle

• NEW: A thread that has not yet started i.e. after creating a thread with new operator
• RUNNABLE: A thread executing in the JVM i.e. after calling start()
• RUNNING: It’s NOT a valid state in Oracle 15 docs
• BLOCKED: A thread that is blocked waiting for a monitor lock is in this state
• WAITING: A thread that is waiting for another thread to perform a particular action
• TIMED_WAITING: A thread that is waiting for specified waiting time
• TERMINATED: A thread that has exited

 Thread Constructors

• Thread()
• Thread​(Runnable target)
• Thread​(Runnable target, String name)
• Thread​(String name)
• Thread​(ThreadGroup group, Runnable target)
• Thread​(ThreadGroup group, Runnable target, String name)
• Thread​(ThreadGroup group, Runnable target, String name, long stackSize)
• Thread​(ThreadGroup group, Runnable target, String name, long stackSize, boolean inheritThreadLocals)
• Thread​(ThreadGroup group, String name)

Runnable Target: Class implementing runnable interface or extending Thread class

Stack Size: approximate number of bytes space that VM should allocate for the thread, on a few OS, it will not have any effect though