Java The Thread class and the Runnable interface

Java’s multi-threading system is built upon the Thread class, its methods, and its companion interface, Runnable. This class belongs to package java.lang and hence there is no need of explicitly importing it.

Thread encapsulates a thread of execution, since you cannot directly refer to the internal state of a running thread, you will deal with it through its proxy, the Thread instance that spawned it. To create a new thread your program will either extend Thread class or implement the Runnable interface.

The Thread class defines several methods that help manage threads:

Java Messaging

When programming with most other languages, you must depend on the O.S. to establish communication between threads. This, of course, adds overhead.

By contrast, Java provides a clean, low-cost way for two or more threads to talk to each other, via calls to predefined methods that all objects have.

Java’s messaging system allows a thread to enter a synchronized method on an object, and then wait there until some other thread explicitly notifies it to come out of wait state.

Java Synchronization

Because multi-threading introduces an asynchronous behaviour to your programs, there must be a way for you to enforce synchronization when you need it.

For example, if you want two threads to communicate and share a complicated data structure, such as a linked list, you need some way to ensure that they do not conflict with each other.

That is, you must prevent one thread from writing data while another thread is in the middle of reading it. Java uses monitor for inter-thread synchronization.

You can think of a monitor as a very small box that can hold only one thread.

Once a thread enters a monitor, all other threads must wait until that thread exits the monitor. In this way, a monitor can be used to protect a shared asset from being manipulated by more than one thread at a time.

Most multi-threaded systems expose monitors as objects that your program must explicitly acquired and lock. Java provides a cleaner solution.

There is no class "monitor", instead, each object has its own implicit monitor that is automatically entered when one of the object's synchronized method is called.

Once a thread is inside a synchronized method no other thread can call any other synchronized method on the same object. This enables you to write very clear and concise multi-threaded code, because synchronization support is built into the language.

Java Thread Priorities

Java assigns to each thread a priority that determines how that thread should be treated with respect to the others.

Thread priorities are integers that specify the relative priority of one thread to another. As an absolute value, a priority is meaningless; a higher-priority thread does not run any faster than a lower-priority thread if it is the only thread running. Instead, a thread’s priority is used to decide when to switch from one running thread to next. This is called a ontext switch.

The rules that determine when a context switch takes place are simple:

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

• A thread can be pre-empted by a higher-priority thread. In this case, a lower priority thread that does not yield the processor is simply pre-empted no matter what it is doing, by a higher priority thread. Basically, as soon as a higher-priority thread wants to run, it does. This is called preemptive multitasking. Some OS support non-preemptive priority based scheduling. In such case a high priority thread gets chance only when low priority thread completes.
• In cases where two threads with the same priority are competing for CPU cycles, the situation is a bit complicated. For OS such as windows 98, threads of equal priority are normally time-sliced automatically in the round-robin fashion. For other types of OS's, thread of equal priority must voluntarily (on their own) yield (give) control to their peers. If they do not, the other threads will not run.

Note: Problems can arise from the differences in the way that O.S.’s context-switch threads of equal priority.

Java Thread Life Cycle (Thread States)

Thread exists in several states. A thread can be running. It can be ready to run as soon as it gets CPU time. A running thread can be suspended, which temporarily suspends its activity. A suspended thread can then be resumed, allowing it to pick up where it left off. A thread can be blocked when waiting for a resource. At any time, a thread can be terminated, which halts its execution immediately. Once terminated a thread cannot be resumed.

1. Newborn State

When we create a thread object, the thread is born and is said to be in newborn state. The thread is not yet scheduled for running. At this state, we can do only one of the following things with it:

1. Schedule it for running using the start() method.

2. Kill it using stop() method

If scheduled, it moves to runnable state. If we attempt to use any other method at this stage, an exception will be thrown.

2. Runnable State

The runnable state means that the thread is ready for execution and is waiting for the availability of the processor. That is, the thread has joined the queue of threads that are waiting for execution. If all threads are of equal priority, then normally they are given time slots for execution in round robin fashion. The thread that relinquishes control joins the queue at the end and again waits for its run. However, if we want a thread to relinquish control to another thread of equal priority before its turn comes, it can do so by invoking the yield() method.

3. Running State

Running means that the processor has given its time to the thread for its execution. The thread runs until it relinquishes control on its own or it is pre-empted by a higher priority thread.

A running thread may relinquish its control in one of the following situations:

• Its time-slice is over
• It is pre-empted by a higher priority thread
• It yields i.e. voluntarily gives up control.
• It has completed its execution
• It is stopped by some other thread.
• It has been suspended using suspend() method; a suspended thread can be revived by using the resume() method. This approach is useful when we want to suspend a thread for some time due to certain reason, but do not want to kill it.
• It has been made to sleep. We can put a thread to sleep for a specified time period using the static method sleep(time) of Thread class where time is in milli-seconds. This means that the thread is out of the queue during the time period. The thread re-enters the runnable state as soon as this time period is elapsed.
• It has been told to wait until some event occurs. This is done using the wait() method. The thread can be scheduled to run again using the notify() method.
• It is performing some I/O operation.

4. Blocked State

A thread is said to be blocked when it is prevented from entering into the runnable state and subsequently the running state. This happens when the thread is suspended, sleeping or waiting in order to satisfy certain requirements. A blocked thread is considered “not runnable” but not dead and therefore fully qualified to run again.

5. Dead State

Every thread has a life cycle. A running thread ends its life when it has completed executing its run() method. It has a natural death. However, we can kill it by sending the stop message to it at any state thus causing a premature death. A thread can be killed as soon as it is born, or while it is running, or even when it is in “not runnable” (blocked condition).