Java ConcurrencyThread Management

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1. Basic thread management2. Thread synchronization mechanisms3. Thread creation and management

delegation with executors4. Fork/Join farmework to enhance the

performance of your application5. Data structures for concurrent programs6. Adapting the default behavior of some

concurrency classes to your needs 7. Testing Java concurrency applications

In this chapter, we will cover:

● Creating and running a thread● Getting and setting thread information● Interrupting a thread● Controlling the interruption of a thread● Sleeping and resuming a thread● Waiting for the finalization of a thread● Creating and running a daemon thread● Processing uncontrolled exceptions in a

thread● Using local thread variables

● Grouping threads into a group● Processing uncontrolled exceptions in a

group of threads● Creating threads through a factory

Creating and running a thread

Threads are Objects. We have two ways of creating a thread in Java:● Extending the Thread class and overriding

the run() method● Building a class that implements the

Runnable interface and then creating an object of the Thread class passing the Runnable object as a parameter

Creates and runs 10 threads.

Each thread calculates and prints the mutiplication table of a number between 1 and 10.

How to do it...

How it works...

System.exit(n)

Runtime.getRuntime().exit(n)0/?(1~127, 128~255, <0)

There’s more...

You can implement a class that extends Thread class and overrides the run() method of this class.Call the start() method to have a new execution thread.

Getting and setting thread information

The Thread class saves some infomation attributes that can help us to identify a thread:● ID: A unique identifier for each Thread.● Name● Proiority: The priority of the Thread

objects.value ∈ [1, 10] (low-->high)

IllegalArgumentExceptionNot recommended to change, but you can use if you want.

● Status: Thread can be in one of these six states (new, runnable, blocked, waiting, time watiing, terminated)

You can’t modify the ID and stauts of a thread.

Develop a program that establishes the name and priority for 10 threads, and then show information about their status until they finish.

How to do it...

New 10 threads and set the priority of them

Create log.txt file record the status of 10 threads, and start them.

If we detect a change in the status of a thread, we write them on the file

The writeThreadInfo method

How it works...

Interrupting a thread

Finish a program:● All its non-daemon threads end its

execution.● One of the threads use the System.exit()

method.

Interruption mechanism

Interruption mechanism

Thread has to:● Check if it has been interrupted or not.● Decide if it responds to the finalization

request or not.

Thread can ignore it and continue with its execution.

Develop a program that creates Thread and, after 5 seconds, will force its finalization using the interruption mechanism.

How to do it...

How it works...

There’s more...

The Thread class has another method to check whether Thread has been interrupted or not.Thread.interrupted()

Difference: isInterrupted() and interrupted()

● interrupted() is static and checks the current thread.

● isInterrupted() is an instance method which checks the Thread object that it is called on.

NOTE:The second one doesn’t change the interrupted attribute value, but the first one set it to false.

Difference: isInterrupted() and interrupted()

isInterrupted(boolean ClearInterrupted)

think about these...

● (1) and (3)● (1) and (4)● (2) and (3)● (2) and (4)

Controlling the interruption of a thread

InterruptedException: A better mechanism to control the interruption of the thread. Detect and throw this exception and catch in the run() method.

We will implement Thread that looks for files with a determined name in afolder and in all its subfolders to show how to use the InterruptedException exceptionto control the interruption of a thread.

How to do it...

How it works...

There’s more...

The InterruptedException exception is thrown by some Java methods related with the concurrency API such as sleep()

Sleeping and resuming a thread

A thread in a program checks a sensor state once per minute. The rest of the time, the thread does nothing. ● Thread.sleep()● TimeUnit.SECONDS.sleep()

We will develop a program that uses the sleep() method to write the actualdate every second.

How to do it...

How it works...

When Thread is sleeping and is interrupted, the method throws an InterruptedExceptionexception immediately and doesn't wait until the sleeping time finishes.

Waiting for the finalization of a thread

We can use the join() method of the Thread class.When we call this method using a thread object, it suspends the execution of the calling thread until the object called finishes its execution.

join()

public final void join() throws InterruptedExceptionWaits for this thread to die.join() ==> join(0)Throws:InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

How to do it...

How it works...

There’s more...

join(long milliseconds)Waits at most millis milliseconds for this thread to die. A timeout of 0 means to wait forever.

join(long milliseconds, long nanos)

If the object thread1 has the code, thread1.join(1000), the thread2 suspends its execution until one of these two conditions is true:● thread1 finishes its execution● 1000 milliseconds have been passed

When one of these two conditions is true, the join() method returns.

What’s the result?

How it works...

Creating and running a daemon thread

Java has a special kind of thread called daemon thread.

What is daemon thread?

Daemon thread

● Very low priority.● Only executes when no other thread of the

same program is running.● JVM ends the program finishing these

threads, when daemon threads are the only threads running in a program.

What does daemon thread used for...

Normally used as service providers for normal threads.Usually have an infinite loop that waits for the service request or performs the tasks of the thread.They can’t do important jobs.

Example: The java garbage collector.

Developing an example with two threads:● One user thread that writes events on a

queue.● One daemon thread that cleans the queue,

removing the events which were generated more than 10 seconds age.

How to do it...

How it works...

If you analyze the output of one execution of the program, you can see how the queue beginsto grow until it has 30 events and then, its size will vary between 27 and 30 events until theend of the execution.

Something wrong?!

Modify the run() method of WriteTask.java ...

How it works...Think of the reason...

How to find out the reason...

● Modify the run() method of WriteTask, make it easy to distinguish each element.

● Add a scanner, list all element of this deque.

Modify the run() method of WriteTask

Add a scanner...

Modify main() method...

How it works...

Find out the reason...

● ArrayDeque: Not thread-safe● We can use LinkedBlockingDeque…Deque<Event> deque = new LinkedBlockingDeque<Event>();

There’s more...

● You only call the setDaemon() method before you call the start() method. Once the thread is running, you can’t modify its daemon status.

● Use isDaemon() method to check if a thread is a daemon thread or a user thread.

Difference between Daemon and Non Daemon thread in Java :

● JVM doesn't wait for any daemon thread to finish before existing.

● Daemon Thread are treated differently than User Thread when JVM terminates, finally blocks are not called, Stacks are not unwounded and JVM just exits.

Processing uncontrolled exceptions in a thread

There are two kinds of exceptions in Java:● Checked exceptions

IOExceptionClassNotFoundExceptionURLReferenceException

● Unchecked exceptionsNumberFormatExceptionClassCastExceptionNullPointExceptionOutOfMemoryError

Exceptions in run() method:

● Checked exception:We have to catch and treat them, because

the run() method doesn't accept a throws clause. ● Unchecked exception:

A mechanism to catch and treat the unchecked exceptions.

The default behaviour is to write the stack trace in the console and exit the program.

We will learn this mechanism using an example.

How to do it...

How it works...

If the thread has not got an uncaught exception handler, the JVM prints the stack trace in the console and exits the program.

There’s more...

● setDefaultUncaughtExceptionHndler()

Establishes an exception handler for all the Thread objects in the application.

When an uncaught exception is thrown in Thread...

JVM looks for…1. The uncaught exception handler of the

Thread objects.2. The uncaught exception handler for

ThreadGroup of the Thread objects.3. The default uncaught exception handler.

None handlers…The JVM writes the stack trace of the exception and exits.

Using local thread variables

One of the most critical aspects of a concurrent application is shared data.

If you change an attribute in a thread, all the threads will be affected by this change.

The Java Concurrency API provides a clean mechanism called thread-local variables with a very good performance.

We will develop a program that has the problem and another program using the thread-local variables mechanism.

How to do it...

How it works...

Each Thread has a different start time but, when they finish, all have the same value in itsstartDate attribute.

How to do it...

How it works...

There’s more...

● protected T initialValue()● public T get()● public void set(T value)● public void remove()● InheritableThreadLocal● childValue()

InheritableThreadLocal

It provides inheritance of values for threads created from a thread.

You can override the childValue() method that is called to initialize the value of the child thread in the thread-local variable.

How to do it...

How it works...

Grouping threads into a group

ThreadGroup: The threads of a group as a single unit.A threadGroup object can be formed by Thread objects and by another ThreadGroup object.A tree structure of threads.

Simulating a search...We will have 5 threads sleeping during a random period of time, when one of them finishes, we are going to interrupt the rest.

How to do it...

SearchTask.java

How it works...

Normal condition...Abnormal condition...

Normal condition

Note:enumerate method:“activeCount”

Abnormal condition

The name attribute changed three times.

threadGroup.list():Only list active thread.

Processing uncontrolled exceptions in a group of threads

Establish a method that captures all the uncaught exceptions thrown by any Thread of the ThreadGroup class.

We will learn to set a handler using an example.

How to do it...

Declare a constructor and override the uncaughtException().

How it works...

Creating threads through a factory

The factory pattern in OO is a creational pattern.Develop an object whose creating other objects of one or servel classes.Instead of using the new operator.Centralize the creation of objects.

Some advantages:

● It's easy to change the class of the objects created or the way we create these objects.

● It's easy to limit the creation of objects for limited resources. For example, we can only have n objects of a type.

● It's easy to generate statistical data about the creation of the objects.

Java provides the ThreadFactory interface to implement a Thread Object factory.

Well will implement a ThreadFactory interface to create Thread objects with a personalized name while we save statistics of the Thread objects created.

How to do it...

How it works...

There’s more...

If you implement a ThreadFactory interface to centralize the creation of threads, you have to review the code to guarantee that all threads are created using that factory.

End

Thank you!