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Advanced Flow of ControlIntroduction

Two additional mechanisms for controlling process execution are exceptions and threads

Lecture focuses on: exception processing catching and handling exceptions creating new exceptions separate process threads synchronizing threads Thread Priority Grouping Threads

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ExceptionsAn exception is an object that describes an

unusual or erroneous situationExceptions are thrown by a program, and

may be caught and handled by another part of the program

A program can therefore be separated into a normal execution flow and an exception execution flow

An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

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Exception Handling

A program can deal with an exception in one of three ways: ignore it handle it where it occurs handle it in another place in the program

The manner in which an exception is processed is an important design consideration

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Exception Handling

If an exception is ignored by the program, the program will terminate and produce an appropriate message

The message includes a call stack trace that indicates on which line the exception occurred

The call stack trace also shows the method call trail that lead to the execution of the offending line

See Zero.javaThe Application

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The try StatementTo process an exception when it occurs, the

line that throws the exception is executed within a try block

A try block is followed by one or more catch clauses, which contain code to process an exception

Each catch clause has an associated exception type

When an exception occurs, processing continues at the first catch clause that matches the exception type

See Adding.java The Application

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Exception Propagation If it is not appropriate to handle the exception

where it occurs, it can be handled at a higher level

Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost level

A try block that contains a call to a method in which an exception is thrown can be used to catch that exception

See Propagation_Demo.javaThe Application

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ExceptionsExceptions can be RuntimeExceptions

(defined in java.lang) or non- Runtime exceptions such IO exceptions

Runtime exceptions are those exceptions that occur within the Java runtime system.

This includes arithmetic exceptions (such as when dividing by zero), pointer exceptions (such as trying to access an object through a null reference), and indexing exceptions (such as attempting to access an array element through an index that is too large or too small).

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ExceptionsAn exception is either checked or uncheckedA checked exception (non-RuntimeExceptions)

can only be thrown within a try block or within a method that is designated to throw that exception.

IO exceptions are checked (IOException classes in java.io and java.net)

Checked exceptions must be listed in the method’s throws clause. Echo.java

The compiler will complain if a checked exception is not specified in the throws clause

An unchecked exception does not require explicit handling, though it could be processed that way

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The throw Statement

A programmer can define an exception by extending the appropriate class

Exceptions are thrown using the throw statement:

throw exception-object;

See Throw_Demo.java The Application

Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown

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The finally Clause

A try statement can have an optional clause designated by the reserved word finally

If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete

Also, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

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ThreadsProcessing can be broken into several separate

threads of control which execute at the same time"At the same time" could mean true parallelism or

simply interlaced concurrent processingA thread is one sequential flow of execution that

occurs at the same time another sequential flow of execution is processing the same program

They are not necessarily executing the same statements at the same time

See the Threads link The life-Cycle of a Thread

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ThreadsA thread can be created by deriving a new

thread from the Thread classThe run method of the thread defines the

concurrent activity, but the start method is used to begin the separate thread process

See Simultaneous.java The ApplicationA thread can also be created by defining a

class that implements the Runnable interfaceBy implementing the interface, the thread

class can be derived from a class other than Thread

See the Threads tutorial link

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Shared DataPotential problems arise when multiple

threads share data

Specific code of a thread may execute at any point relative to the processing of another thread

If that code updates or references the shared data, unintended processing sequences can occur that result in incorrect results

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Shared DataConsider two withdrawals from the same

bank account at the same time

task: withdraw 300

Is amount <= balance

YES

balance -= 300

balance

531

231

-69

task: withdraw 300

Is amount <= balance

YES

balance -= 300

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SynchronizationMultiple threads of control can be made safe if

areas of code that use shared data are synchronized

When a set of code is synchronized, then only one thread can be using that code at a time

The other threads must wait until the first thread is complete

This is an implementation of a synchronization mechanism called a monitor

See ATM_Accounts.java The Application

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Controlling Threads

Thread processing can be temporarily suspended, then later resumed, using methods from the Thread class

A thread can also be put to sleep for a specific amount of time

These mechanisms can be quite helpful in certain situations, like controlling animations

See Bouncing_Ball2.java The Applet

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Thread Priority The Java runtime supports a very simple,

deterministic CPU scheduling algorithm known as fixed priority scheduling

This algorithm schedules threads based on their priority relative to other runnable threads

When a Java thread is created, it inherits its priority from the thread that created it

Thread priorities are integers ranging between MIN_PRIORITY (0) and MAX_PRIORITY (10)

You can modify a thread's priority at any time after its creation using the setPriority method of the Thread class

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Thread Prioritythe runtime system chooses the runnable

thread with the highest priority for execution If two threads of the same priority are

waiting for the CPU, the scheduler chooses one of them to run in a round-robin fashion

The chosen thread will run until one of the following conditions is true: A higher priority thread becomes runnable. It yields, or its run method exits. On systems that support time-slicing, its

time allotment has expired

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Thread PriorityThe Java runtime system's thread scheduling

algorithm is also preemptive. If at any time a thread with a higher priority than all other runnable threads becomes runnable, the runtime system chooses the new higher priority thread for execution

you should try to write "well-behaved" threads that voluntarily relinquish the CPU periodically and give other threads an opportunity to run

you should never write Java code that relies on time-sharing--this will practically guarantee that your program will give different results on different computer systems.

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Synchronizing Threads The Producer/Consumer ExampleUsing the notifyAll and wait Methods Avoid Starvation and Deadlock

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Grouping Threads Every Java thread is a member of a thread group Thread groups provide a mechanism for

collecting multiple threads into a single object and manipulating those threads all at once, rather than individually

the runtime system automatically places the new thread in the same group as the thread that created it (known as the current thread group

When a Java application first starts up, the Java runtime system creates a ThreadGroup named main

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Grouping Threads If you create a thread within an applet, the

new thread's group may be something other than main, depending on the browser or viewer that the applet is running

Many browsers allocate a thread for each applet on a page. Some browsers allocate a thread group for each applet, so that it's easy to kill all the threads that belong to a particular applet

In any case, you're guaranteed that every thread that any of an applet's major milestone methods creates belongs to the same thread group

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Grouping Threads Creating a Thread Explicitly in a Group

ThreadGroup myThreadGroup = new ThreadGroup("My Group of Threads");

Thread myThread = new Thread(myThreadGroup, "a thread for my group");

Methods that Operate on All Threads within a

GroupThe ThreadGroup class has three methods that allow you to modify the current state of all the threads within that group:

• resume • stop • suspend

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The hierarchy of threads and thread groups