com.databasesandlife.util

Class ThreadPool

java.lang.Object

com.databasesandlife.util.ThreadPool

public class ThreadPool
extends Object

Runs a number of Runnable tasks in a number of threads (over a number of CPU cores).

Usage:

      ThreadPool pool = new ThreadPool();

      // optional
      pool.setThreadNamePrefix("foo"); // for debugger output
      pool.setThreadCount(5);

      // add one or more seed tasks
      pool.addTask(new Runnable() { ... });

      // start threads, execute the seed tasks, and execute any tasks they create
      pool.execute();
 

In the case that any task throws an exception, this exception is thrown by the execute() method. If all tasks run to completion, the execute() method returns with no value.

Tasks can depend on other tasks. Use the addTaskWithDependencies(List, Runnable) method to add a new task, which will only start in the first parameter after all the tasks in the second parameter have run to completion. All tasks in the List should have been previously added using the normal addTask(Runnable...) method, or themselves with addTaskWithDependencies(List, Runnable).

The difference to an ExecutorService is:

• The processing of tasks can add additional tasks to the queue (whereas to an ExecutorService the client adds a fixed number of tasks, and they are then executed, without the executing tasks being able to add more tasks). Such functionality is mandatory for web crawlers, which, during the processing of pages, discover links which point to additional pages which require processing.
  
  
• It is impossible to forget to "shutdown" a ThreadPool and cause a leakage of threads, as is easily possible with ExecutorService. If the execute() method is never called then no threads are ever started and the object can be garbage collected normally. If the execute() method is called then that method makes sure all threads it creates are destroyed.

Author:

This source is copyright Adrian Smith and licensed under the LGPL 3.

See Also:

Project on GitHub

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected class	ThreadPool.RunnerRunnable
protected static class	ThreadPool.TaskWithDependencies

Field Summary

Fields

Modifier and Type	Field and Description
protected Map<Runnable,List<ThreadPool.TaskWithDependencies>>	blockerTasks
protected Exception	exceptionOrNull
protected com.databasesandlife.util.IdentityHashSet<Runnable>	executingTasks
protected com.databasesandlife.util.IdentityHashSet<Runnable>	readyTasks
protected int	threadCount
protected String	threadNamePrefix

Constructor Summary

Constructors

Constructor and Description
ThreadPool()

Method Summary

Modifier and Type	Method and Description
void	addTask(Runnable... tasks)
void	addTasks(Collection<Runnable> tasks)
void	addTaskWithDependencies(List<Runnable> dependencies, Runnable r)
void	execute()
void	setThreadCount(int count)
void	setThreadNamePrefix(String prefix)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

threadNamePrefix

@Nonnull
protected String threadNamePrefix

threadCount

protected int threadCount

readyTasks

protected final com.databasesandlife.util.IdentityHashSet<Runnable> readyTasks

executingTasks

protected final com.databasesandlife.util.IdentityHashSet<Runnable> executingTasks

blockerTasks

protected final Map<Runnable,List<ThreadPool.TaskWithDependencies>> blockerTasks

exceptionOrNull

@CheckForNull
protected Exception exceptionOrNull

Constructor Detail

ThreadPool

public ThreadPool()

Method Detail

setThreadCount

public void setThreadCount(int count)

setThreadNamePrefix

public void setThreadNamePrefix(String prefix)

addTaskWithDependencies

public void addTaskWithDependencies(List<Runnable> dependencies,
                                    Runnable r)

addTask

public void addTask(Runnable... tasks)

addTasks

public void addTasks(Collection<Runnable> tasks)

execute

public void execute()