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Java threads FAQ

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Concurrent programming features

Q: What is threaded programming and when is it used?

A: Threaded programming is normally used when a program is required to do more than one task at the same time. Threading is often used in applications with graphical user interfaces; a new thread may be created to do some processor-intensive work while the main thread keeps the interface responsive to human interaction.

The Java programming language has threaded programming facilities built in, so it is relatively easy to create threaded programs. However, multi-threaded programs introduce a degree of complexity that is not justified for most simple command line applications.

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Q: What is a thread?

A: In Java the Thread class represents a single independent path of execution in a Java Virtual Machine. When you run a Java program it implicitly starts a single thread of execution. The Thread class enables programmers to create additional threads and set them running. A number of threads may run in parallel, but only one is actively executed at a given moment.

The Java runtime system uses fairly complex thread scheduling mechanisms to coordinate the execution of threads, but this does not require privileged knowledge or detail level intervention by programmers. Programmers can manage the high level creation, initiation and distribution of tasks amongst threads through simple API methods.

The example below shows the simplest approach to thread creation and task execution; construct a new Thread with a Runnable argument and start it.

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What is a thread?

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Q: Which class is the wait() method defined in?

A: The wait() method is defined in the Object class, which is the ultimate superclass of all others. So the Thread class and any Runnable implementation inherit this method from Object. The wait() method is normally called on an object in a multi-threaded program to allow other threads to run. The method should only be called by a thread that has ownership of the object's monitor, which usually means it is in a synchronized method or statement block.

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Which class is the wait() method defined in?

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Q: Which class is the sleep(long) method defined in?

A: The sleep(long) method is defined as a static method of the Thread class for two reasons. First, it is responsible for delaying the execution of threads rather than any other objects, so Thread is the logical host for the method. Secondly, it is static because it suspends the currently executing thread, which is implicitly the thread that called the method. The programmer does not need to know which other threads are alive.

The sleep(long) method retains the synchronization locks of the current thread, so it can have a significant effect on the behaviour of multi-threaded applications. If the exact period of the delay you require is not important, it may be better to use the wait(long) method.

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Q: Why are wait(), notify() and notifyall() methods defined in the Object class?

A: These methods are detailed on the Java Software Development Kit JavaDoc page for the Object class, they are to implement threaded programming for all subclasses of Object.

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Q: Why are there separate wait and sleep methods?

A: The static Thread.sleep(long) method maintains control of thread execution but delays the next action until the sleep time expires. The wait method gives up control over thread execution indefinitely so that other threads can run.

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Why are there separate wait and sleep methods?

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Threads and runnable types

Q: What's the difference between Thread and Runnable types?

A: A Java Thread controls the main path of execution in an application. When you invoke the Java Virtual Machine with the java command, it creates an implicit thread in which to execute the main method. The Thread class provides a mechanism for the first thread to start-up other threads to run in parallel with it.

The Runnable interface defines a type of class that can be run by a thread. The only method it requires is run, which makes the interface very easy to fulfil by extending existing classes. A runnable class may have custom constructors and any number of other methods for configuration and manipulation.

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Q: How does the run() method in Runnable work?

A: It may help to think of the run method like the main method in standard single threaded applications. The run method is a standard entry point to run or execute a class. The run method is normally only executed in the context of an independent Thread, but is a normal method in all other respects.

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How does the run() method in Runnable work?

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Q: A Thread is runnable, how does that work?

A: The Thread class' run method normally invokes the run method of the Runnable type it is passed in its constructor. However, it is possible to override the thread's run method with your own.

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A Thread is runnable, how does that work?

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Q: Why not override Thread to make a Runnable?

A: There is little difference in the work required to override the Thread class compared with implementing the Runnable interface, both require the body of the run() method. However, it is much simpler to make an existing class hierarchy runnable because any class can be adapted to implement the run() method. A subclass of Thread cannot extend any other type, so application-specific code would have to be added to it rather than inherited.

Separating the Thread class from the Runnable implementation also avoids potential synchronization problems between the thread and the run() method. A separate Runnable generally gives greater flexibility in the way that runnable code is referenced and executed.

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Q: What's the difference between a thread's start() and run() methods?

A: The separate start() and run() methods in the Thread class provide two ways to create threaded programs. The start() method starts the execution of the new thread and calls the run() method. The start() method returns immediately and the new thread normally continues until the run() method returns.

The Thread class' run() method does nothing, so sub-classes should override the method with code to execute in the second thread. If a Thread is instantiated with a Runnable argument, the thread's run() method executes the run() method of the Runnable object in the new thread instead.

Depending on the nature of your threaded program, calling the Thread run() method directly can give the same output as calling via the start() method, but in the latter case the code is actually executed in a new thread.

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Q: Can I implement my own start() method?

A: The Thread start() method is not marked final, but should not be overridden. This method contains the code that creates a new executable thread and is very specialised. Your threaded application should either pass a Runnable type to a new Thread, or extend Thread and override the run() method.

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Multi-threaded design questions

Q: Do I need to use synchronized on setValue(int)?

A: It depends whether the method affects method local variables, class static or instance variables. If only method local variables are changed, the value is said to be confined by the method and is not prone to threading issues.

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Do I need to use synchronized on setValue(int)?

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Q: How do I create a Runnable with inheritance?

A: To introduce a Runnable type to an existing class hierarchy, you need to create a sub-class that declares that it implements the Runnable interface, and provide a run method to fulfil the interface. This combination of interface and inheritance means that runnable implementations can be very minor extensions of existing classes, as in the example below...

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Q: What is the volatile modifier for?

A: The volatile modifier is used to identify variables whose values should not be optimised by the Java Virtual Machine, by caching the value for example. The volatile modifier is typically used for variables that may be accessed or modified by numerous independent threads and signifies that the value may change without synchronization.

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Q: What is the SwingUtilities.invokeLater(Runnable) method for?

A: The static utility method invokeLater(Runnable) is intended to execute a new runnable thread from a Swing application without disturbing the normal sequence of event dispatching from the Graphical User Interface (GUI). The method places the runnable object in the queue of Abstract Windowing Toolkit (AWT) events that are due to be processed and returns immediately. The runnable object's run() method is only called when it reaches the front of the queue.

The deferred effect of the invokeLater(Runnable) method ensures that any necessary updates to the user interface can occur immediately, and the runnable work will begin as soon as those high priority events are dealt with. The invoke later method might be used to start work in response to a button click that also requires a significant change to the user interface, perhaps to restrict other activities, while the runnable thread executes.

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Q: I get "incompatible return type" for my thread's getState( ) method!

A: It sounds like your application was built for a Java software development kit before Java 1.5. The Java API Thread class method getState() was introduced in version 1.5. Your thread method has the same name but different return type. The compiler assumes your application code is attempting to override the API method with a different return type, which is not allowed, hence the compilation error.

You have two main options; compile your program with a JSDK before version 1.5 or rename your method and adapt any client classes to the new name.

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Multi-threaded design patterns

Q: What is a working thread?

A: A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them.

The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.

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Thread programming jargon

Q: What is a green thread?

A: A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads.

There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.

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Q: What are native operating system threads?

A: Native operating system threads are those provided by the computer operating system that plays host to a Java application, be it Windows, Mac or GNU/Linux. Operating system threads enable computers to run many programs simultaneously on the same central processing unit (CPU) without clashing over the use of system resources or spending lots of time running one program at the expense of another. Operating system thread management is usually optimised to specific microprocessor architecture and features so that it operates much faster than Java green thread processing.

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Thread programming problems

Q: Apache Log4J has thrown a ThreadDeath error!

A: The java.lang.ThreadDeath type signals a serious Error in the execution of a Java program and should not normally be caught by applications, it should be left to escalate and terminate the application. ThreadDeath is a Throwable type, so the Apache logging package may blindly catch Throwable rather than an Exception, which is not recommended. More likely ThreadDeath is caught so that other asynchronous threads may be cleaned up before logging is terminated.

Since this ThreadDeath instance is wrapped in the commons LogConfigurationException, the root cause of the problem is most likely that "a suitable LogFactory or Log instance cannot be created by the corresponding factory methods". In other words your logging configuration declares a type that cannot be instantiated, possibly because of bad spelling, ultimately because the named class is not available to the relevant classloader.

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