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Threads

 


 

Some Important Concepts

 

Why multi-threading:

Multi-threading makes a program more efficient by minimizing the idle CPU time. It is especially relevant in an interactive program, or in any program that has to wait for some event to occur or for some resource, to proceed further. While one thread of the program waits, another thread can continue executing code that is not dependent on that event to occur.

Thread priorities:

Thread priorities are integers that specify the relative priority of one thread to another. These are used by the thread scheduler to decide when each thread should be allowed to run. (Thread scheduler is a piece of system code . The scheduler might be part of JVM or host operating system. It determines which thread is actually running on each available CPU at a given time.) A thread’s priority is used to decide when to switch from one running thread to the next. This is called a context-switch. The rules that determine when a context switch takes place are:

  • A thread can voluntarily relinquish control. This is done by explicitly yielding, sleeping or blocking on pending I/O. In this scenario, all other threads are examined, and  the highest priority thread that is ready to run is given the CPU.

  • A thread can be preempted by a higher priority thread. In this case, a lower priority thread that does not yield the processor is simply preempted – no matter what it is doing – by a higher priority thread. This is called preemptive multithreading.

In cases where two threads with the same priority are competing for CPU cycles, the outcome is system dependent. In operating systems like windows, equal priority threads are time-sliced automatically in round robin fashion. If there is more than one waiting thread, scheduler chooses one of them. There is no guarantee that the longest waiting thread will be chosen. For other types of operating systems, such as Solaris, threads of equal priority must voluntarily yield control to their peers. If they do not, other threads will not run. 

The Java specification states that threads must have priorities, but it does not dictate precisely what the scheduler do about priorities. This vagueness is a problem: algorithms that rely on manipulating thread priorities might not run consistently on all platforms. If you want smooth multi-tasking, you should not rely on priorities of threads. Also some types of tasks are CPU intensive. Such threads dominate the CPU. For these types of threads, it is better to yield control occasionally, so that other threads can run too.

The value of thread priorities must be within the range MIN_PRIORITY (value=1) and MAX_PRIORITY (value=10). The default is NORM_PRIORITY (value=5). 

All newly created threads have their priority set to that of the creating thread.

Main thread:

When a Java program starts up, one thread begins running immediately. This thread is the one that is executed when your program begins. The main thread is important for two reasons:

  • It is the thread from which other child threads will be spawned.

  • It must be the last thread to finish execution. When the main thread stops, your program terminates.

The main thread is created automatically when a program is started. It can be controlled through a Thread object. To do so, you must obtain a reference to it by calling the method currentThread(), which is a public, static member of Thread.

Thread t=Thread.currentThread();

When t is used as an argument to println(), it displays the name of the thread, its priority and the name of its group in that order.

A thread group is a data structure that controls the state of a collection of threads as a whole. This process is managed by the particular runtime environment.

Return of run (): 

When the run() returns, the thread has finished its task and is considered dead. There is no way to restart it. If you want the thread’s task to be performed again, you have to construct and start a new thread instance. The dead thread continues to exist; it is an object like any other object, and you can still access its data and call its methods. You just cannot make it run again.

Instead of using stop() of thread class, if a thread might need to be killed from another thread, then you should send it an interrupt() from the killing method.

Thread states: 

When you call start() on a thread, the thread does not run immediately. It goes into a ready-to-run state and stays there until the scheduler moves it to the “running state”. Then the run() is called. In the course of executing run(), the thread may temporarily give up the CPU and enter some other state for a while. The thread states are:

  • Running

  • Waiting

  • Sleeping

  • Suspended

  • Blocked

  • Dead

  

Controlling threads

Yielding  

A call to the yield() method causes the currently executing thread to move to the ready state if the scheduler is willing to run any other thread in its place. There are two possible scenarios. If any other threads are in the ready state, then the thread that just yielded might have to wait a while before it gets to execute again. However, if there are no other waiting threads, then the yielding thread gets to continue executing immediately. Most thread schedulers do not stop the yielding thread from running in favor of a lower priority thread. The yield() always causes the currently executing thread to yield. Yielding allows a time consuming thread to permit other threads to execute.

Suspending 

Suspending a thread is a mechanism that allows any arbitrary thread to make another thread un-runnable for an indefinite period of time. The suspended thread becomes Runnable when some other thread resumes it. It is very easy to cause deadlock in a program using these methods, since a thread does not have any control over when it is suspended and it might be in a critical section, holding an object lock at the time. The exact effect of suspend() and resume() is much better implemented using wait() and notify().

Sleeping 

A sleeping thread passes time without doing anything and without using the CPU. A call to sleep() requests the currently executing thread to cease executing for (approx) a specified amount of time. The method sleep(), like yield(), is static. Both operate on currently executing thread.

When a thread has finished sleeping, it does not continue execution. It moves to a ready state and waits for the scheduler’s permission.

The Thread class has a method called interrupt(). A sleeping thread that receives an interrupt() call moves immediately into ready state; when it gets to run, it will execute its InterruptedException handler.

Blocking 

Many methods that perform input/output have to wait for some occurrence in the outside world before they can proceed; this behavior is known as blocking. In general, if a method needs to wait an indefinite time until some I/O takes place, then a thread executing that method should graciously step out of the Running state. All Java I/O methods behave this way. A thread that has stepped out in this fashion is said to be blocked.

Generally, if you see a method with a name that suggests that it might do nothing until something becomes ready, you should expect that the caller thread might be blocked becoming un-runnable and losing the CPU, when the method is called.

A thread can also become blocked if it fails to acquire the lock for a monitor or if it issues a wait() call. Internally, most blocking for I/O, is implemented using wait() and notify().

Important  Exceptions:

IllegalThreadStateException: thrown to indicate that a thread is not in an appropriate state for the requested operation. 

IllegalMonitorStateException: thrown to indicate that a thread has attempted to wait on an object’s monitor without owning the specified monitor. 

Why stop (), suspend () and resume () are deprecated?  

stop () is inherently unsafe. Stopping a thread causes it to unlock all the monitors that it has locked. (the monitors are unlocked as the ThreadDeath exception propagated up the stack.) If any of the objects previously protected by these monitors were in an inconsistent state, other threads may now view these objects in an inconsistent state. Such objects are said to be damaged. When threads operate on damaged objects, arbitrary behavior can result. This behavior may be subtle and difficult to detect, or it may be pronounced. Unlike other unchecked exceptions, ThreadDeath kills threads silently; thus, the user has no warning that his program may be corrupted. The corruption can manifest itself at any time after the actual damage occurs, even after hours or days in the future. Most uses of stop () should be replaced by code that simply modifies some variable to indicate that the target thread should stop running. The target thread should check this variable regularly, and return from its run method in an orderly fashion if the variable indicates that it is to stop running. To ensure prompt communication of the stop () request, the variable must be volatile (or access to the variable must be synchronized. 

suspend () is inherently deadlock-prone. If the target thread holds a lock on the monitor protecting a critical system resource when it is suspended, no thread can access this resource until the target thread is resumed. If the thread that would resume the target thread attempts to lock this monitor prior to calling resume (), deadlock results. Such deadlocks typically manifest themselves as frozen processes. 

resume () exists solely for suspend (). Therefore it has been deprecated too.


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