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Threading in java - a pragmatic primer


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Yet another threading primer in java;
with best practices & pragmatic tips;

Published in: Technology
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Threading in java - a pragmatic primer

  1. 1. SivaramaSundar.D 29 th Nov 2012
  2. 2.  Concepts When & Why do we need threads Threads in java Concurrency: Thread control & Synchronization Concurrency: Data management between threads Best practices: Threading the right way; Q&A
  3. 3.  Processes  A Process has a self-contained execution environment; an application, in general terms – with own memory address space; with a main execution thread; which can own O/S resource handles – files, sockets etc.  1..* threads; Each process has one Main thread;  System threads – GC, Object finalization, JVM housekeeping  Timers and User created threads Threads  Execution unit – to execute a sequence of instructions  Owns: Stack, Program Counter, Local variables  Shares: Memory, Filehandles, Process States ThreadGroups  Grouping threads into a logical collection; Not used much. ThreadPools  A thread pool is a collection of threads set aside for a specific task; Ex: webserver thread pool; saves thread creation overheads everytime;  execute(Runnable command)  Used for executing large numbers of asynchronous tasks  provide a boundary mechanism to create and managing the resources within the pool  Better thread management; Cleaner shutdown of threads;  Ability to Add, Remove, Enumerate future tasks; Apt for a scheduler;
  4. 4.  Multitasking – Receive data via a socket & write to file(s) A Server handling multiple concurrent requests to serve data Make the UI more responsive Number crunching; Bulk data processing; Take advantage of multiprocessor systems Simplify program logic when there are multiple independent entities Perform blocking I/O without blocking the entire program Ex:  A Webserver  A real time device monitor to display device parameters  A Monitoring application, polling multiple sources & providing live updates
  5. 5.  Runnable Interface & Thread Class, Daemon threads  Instantiate the “Thread” Class with a “Runnable” implementation (preferred way!)  Subclass the “Thread” class & override the “run” method Start – begin execution setDaemon – thread will be terminated by VM during shutdown; normal threads won’t; sleep  Sleeps are not precise;  Sleep either in ms or ns  The Sleep can be interrupted, by other threads via the thread.interrupt call Yield (rarely used) - Relinquish control ; during long running operations; Interrupt  Interrupts the wait state of the thread; invoked by the thread owner;  Raises a InterruptedException
  6. 6.  Join  Makes the calling thread wait until other thread completes;  Typical usage: make sure all the child threads are terminated;  Can be interrupted by the thread.interrupt call wait Notify – Wakes up the thread waiting on the given object’s monitor notifyAll – Wakes up all the threads waiting on the given object’s monitor Obselete methods  suspend  resume  Stop – use internal flags, join, wait & interrupt mechanisms instead
  7. 7.  Timers, TimerTask (Daemon)  Schedule tasks (Runnable) for future execution in a background thread. Tasks may be scheduled for one-time execution, or for repeated execution at regular intervals  Schedule (task, delay) ThreadFactory  Help create threads of a given type; subclassed threads ThreadInfo  Contains the information about a thread ThreadReference  Object ref. with additional access to thread-specific information from the target VM. Provides access to internal stack frames, monitor references.
  8. 8.  Why Synchronization  Prevent shared data corruption / thread interference / data integrity Code  Locks (Monitors)- Synchronized, Volatile  Each object in java has a unique monitor. When a synchronized method / block is invoked by the thread, the thread tries to take ownership of the monitor or block until it gets the ownership; The Thread acquires the monitor for the given object (ex:this / method, class object ref.). A monitor is automatically released when the method / block execution completes.  Only one thread at a time can own an objects monitor.  Synchronized  Protect Code & Make data changes visible  Block level  Method level (uses intrinsic lock of the method’s object instance)  Volatile – bypass processer cache to use main memory  One thread – One Lock – anytime  Locks will be released in the event of any uncaught exceptions  Lock Interface for better control than “Synchronized”  A single Lock can have multiple Conditions  ReentrantLock – lock() ; Try... Finally{ unlock(); }  ReentrantReadWriteLock - to get a read / write or both locks
  9. 9.  Data  Semaphores– Semaphore (Counting Semaphore)  acquire(), release()  Mechanism to control access to a pool of shared resource, between multiple processes, threads  Acts like a gate – for a limited access pool / lift – with a fixed capacity; some threads have to yield control, for the other threads to access the shared data;  While Locks are exclusive, semaphores are not  Other examples:  Fixed no. of meeting rooms – with controlled access  Mutexes – Same as a binary semaphore (lock - yes/no), but across processes  Normally mutexes has owners  Typical usage – to ensure single instance of an application / process
  10. 10.  ThreadLocal<T>  Provide Local variables for the thread (can be accessed only within this thread)  Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive  When a thread dies; the thread local variables are subject to GC Java.utils.concurrent  ThreadPoolExecutor, ScheduledThreadPoolExecutor  Java.util.Collections classes with built in support for handling concurrency & access from multiple threads; uses collection segmentation & hence supports non-blocking – ex: the whole collection is not locked;  ConcurrentHashMap  ConcurrentLinkedDeque  ConcurrentLinkedQueue  ConcurrentSkipListMap  ConcurrentSkipListSet Make normal collections thread safe via - java.util.Collections methods; blocking – ex: whole collection is locked;  SynchronousQueue  SynchronizedCollection  SynchronizedSet  SynchronizedList  SynchronizedMap  SynchronizedSortedSet  SynchronizedSortedMap
  11. 11.  Deadlock  T1 -> W1; T1.DoSomething waits for W2.  T2 -> W2; T2.DoSomething waits for W1.  Hard to debug – but jConsole, jStack helps (demo with jConsole);  Simplify locks / avoid arbitrary synchronization Race Condition  A race condition occurs when 2 or more threads access shared data and they try to change it at the same time;  problems occur when one thread does a "check-then-act" and another thread does something to the value in between the "check" and the "act“; tip: avoid ‘check & act’ situations when using threading;
  12. 12.  White-boarding & Brainstorming Document / Comment all threading code; Be Aware of the synchronized keyword used as part of the method name – it is easy to miss if that a synchronized method uses an intrinsic lock; synchronized blocks are easier to spot; Thorough Code Reviews Use locks judiciously – lock while writes Wait for spawned threads to complete, or force stop Exception handling – a thread will terminate on an unhandled exception Make use of Thread.setDefaultUncaughtExceptionHandler -handler for all threads in the VM or setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh) - handler for single specific thread Use immutable classes – Ex: String, Integer, BigDecimal, as they make concurrency handling simple Know when JVM performs the synchronization for you: Static Initializer, final fields, Creating objects before threads Avoid nested locks; to prevent deadlocks;
  13. 13.  Dont invoke methods on other objects while holding a lock. (Sounds crazy; ) Ensure that when you acquire multiple locks, you always acquire the locks in the same order in all threads. Keep the synchronized blocks as short as possible; Don’t use blocking code inside a synchronized block – ex: Don’t tamper thread priorities; leave it to the JVM & O/S Avoid starvation of resources; Don’t code long running threads; Aids in debugging threading issues:  Thread.holdsLock (Object lockObj)- true if lock is held  Thread.dumpStack()  Inspect using Thread.State / getState()  Provide a thread name when creating a thread  Logs – with thread id’s;  ThreadInfo class  Threaddumps - Provide a stack trace of all running threads  (tool from jdk) jstack <pid> >> threaddumps.log  (alternate) use jConsole to monitor the jvm & analyze stack trace of all live threads  Using the “SendSignal.exe” to send a “break” signal to the process to get a thread dump
  14. 14.  Threading explained in simple terms- Java Concurrency in Practice – Book – Hardcode multi-threading in java - Analyzing thread dumps- dump.html
  15. 15.    