Multithreading: Key Concepts

• Daemon Thread
• If a daemon thread creates a new thread; new thread will be by default daemon
• Daemon thread is low priority thread which runs in background 
• It provides support & services to running non-daemon threads
• For example, GC or finalizer, etc.
• Once all user threads are terminated, JVM will terminate the program by terminating any running daemon threads
• Thread Priorities
• New thread created will by default have priority of creating thread
• Priority is represented by a number between 1 to 10
• 1 = MIN_PRIORITY, 5 = NORM_PRIORITY and 10 = HIGH_PRIORITY
• Thread Interruption
• If any thread is in sleeping or waiting state, calling interrupt() method on thread, breaks the thread state and throws InterruptedException
• If the thread is not in waiting state, method will not throw exception but will set interrupted flag to true
• ThreadLocal
• All threads of an Object share it’s variables, so the variables are not thread safe
• We can achieve thread safety by synchronization, but if we want to avoid synchronization, we can use ThreadLocal
• Every thread has its own ThreadLocal which they can use to set() & get() for the values of static private variables local to the Thread
• Thread Dump
• Is useful for analyzing bottlenecks and deadlock situations in application
• There are different ways to generate ThreadDump
• VisualVM Profiler
• Jstack: jstack pid >> file.tdump
• Kill - PID: on console output i.e. System.out
• jcmd: java 8 & above
• ThreadMXBean
• It is an interface which helps in detecting deadlocks
• Thread.getThreadMXBean().findMonitorDeadlockedThreads()
• Avoid Deadlocks
• Avoid nested locks
• Lock only what is required: field => block => method => object
• Avoid waiting indefinitely
• Follow Lock Orders: Thread2 should acquire locks in same order as Thread1
• Use join() method: sequential execution of threads