Chapter 29 Multithreading
1
Threads Concept
Multiple Thread 1
threads on
Thread 2
multiple
Thread 3
CPUs
Multiple Thread 1
threads
Thread 2
sharing a
Thread 3
single CPU
2
Using the Runnable Interface to Create
and Launch Threads
• Objective: Define a Task, create objects of Task and
run in threads:
• MyTask prints a text and sleeps for a while
Interface Runnable {
void run();
}
class MyTask implements Runnable {
}
3
Creating Tasks and Threads
java.lang.Runnable TaskClass // Client class
public class Client {
...
// Custom task class public void someMethod() {
public class TaskClass implements Runnable { ...
... // Create an instance of TaskClass
public TaskClass(...) { TaskClass task = new TaskClass(...);
...
} // Create a thread
Thread thread = new Thread(task);
// Implement the run method in Runnable
public void run() { // Start a thread
// Tell system how to run custom thread thread.start();
... ...
} }
... ...
} }
4
The Thread Class
«interface»
java.lang.Runnable
java.lang.Thread
+Thread() Creates a default thread.
+Thread(task: Runnable) Creates a thread for a specified task.
+start(): void Starts the thread that causes the run() method to be invoked by the JVM.
+isAlive(): boolean Tests whether the thread is currently running.
+setPriority(p: int): void Sets priority p (ranging from 1 to 10) for this thread.
+join(): void Waits for this thread to finish.
+sleep(millis: long): void Puts the runnable object to sleep for a specified time in milliseconds.
+yield(): void Causes this thread to temporarily pause and allow other threads to execute.
+interrupt(): void Interrupts this thread.
5
The Static yield() Method
You can use the yield() method to temporarily release time
for other threads. For example, suppose you modify the
code in Lines 53-57 in TaskThreadDemo.java as follows:
public void run() {
for (int i = 1; i <= lastNum; i++) {
System.out.print(" " + i);
Thread.yield();
}
}
Every time a number is printed, the print100 thread is
yielded. So, the numbers are printed after the characters.
6
The Static sleep(milliseconds) Method
The sleep(long mills) method puts the thread to sleep for the
specified time in milliseconds. For example, suppose you modify the
code in Lines 53-57 in TaskThreadDemo.java as follows:
public void run() {
for (int i = 1; i <= lastNum; i++) {
System.out.print(" " + i);
try {
if (i >= 50) Thread.sleep(1);
}
catch (InterruptedException ex) {
}
}
}
Every time a number (>= 50) is printed, the print100 thread is put to
sleep for 1 millisecond.
7
The join() Method
You can use the join() method to force one thread to wait for another
thread to finish. For example, suppose you modify the code in Lines
53-57 in TaskThreadDemo.java as follows:
public void run() { Thread Thread
Thread thread4 = new Thread( print100 printA
new PrintChar('c', 40));
thread4.start(); -char token -char token
try {
+getToken +getToken
for (int i = 1; i <= lastNum; i++) { printA.join()
+setToken +setToken
System.out.print(" " + i); +paintCompo +paintCompo
if (i == 50) thread4.join(); Wait for printA
-char
net token net
} to finish+mouseClicke +mouseClicke
} +getToken
d d
catch (InterruptedException ex) { +getToken +setToken printA finished
} +setToken +paintCompone
t
+paintComponet -char token
} +mouseClicked
The numbers after 50 are printed after thread printA is finished.
8
Thread States
A thread can be in one of five states:
New, Ready, Running, Blocked, or
Finished.
yield(), or Running
time out run() returns
Thread created start()
New Ready run() join() Finished
interrupt() sleep()
Target wait()
finished
Wait for target Wait for time Wait to be
to finish out notified
Time out notify() or
notifyAll()
Blocked
Interrupted()
9
isAlive(), interrupt(), and isInterrupted()
The isAlive() method is used to find out the state of a
thread. It returns true if a thread is in the Ready, Blocked,
or Running state; it returns false if a thread is new and has
not started or if it is finished.
The interrupt() method interrupts a thread in the
following way: If a thread is currently in the Ready or
Running state, its interrupted flag is set; if a thread is
currently blocked, it is awakened and enters the Ready
state, and an java.io.InterruptedException is thrown.
The isInterrupt() method tests whether the thread is
interrupted.
10
Thread Priority
• Each thread is assigned a default priority of
Thread.NORM_PRIORITY. You can reset the
priority using setPriority(int priority).
• Some constants for priorities include
Thread.MIN_PRIORITY
Thread.MAX_PRIORITY
Thread.NORM_PRIORITY
11
Thread Synchronization
A shared resource may be corrupted if it is
accessed simultaneously by multiple threads. For
example, two unsynchronized threads accessing
the same bank account may cause conflict.
Step balance thread[i] thread[j]
1 0 newBalance = bank.getBalance() + 1;
2 0 newBalance = bank.getBalance() + 1;
3 1 bank.setBalance(newBalance);
4 1 bank.setBalance(newBalance);
12
Example: Showing Resource Conflict
• Objective: Write a program that demonstrates the problem of
resource conflict. Suppose that you create and launch one
hundred threads, each of which adds a penny to an account.
Assume that the account is initially empty.
java.lang.Runnable
-char token
100 1 1 1
AddAPennyTask
+getToken AccountWithoutSync Account
+setToken
+paintComponet -bank: Account -balance: int
+mouseClicked
+run(): void -thread: Thread[]
+getBalance(): int
+deposit(amount: int): void
+main(args: String[]): void
AccountWithoutSync
Run
13
Race Condition
What, then, caused the error in the example? Here is a possible scenario:
Step balance Task 1 Task 2
1 0 newBalance = balance + 1;
2 0 newBalance = balance + 1;
3 1 balance = newBalance;
4 1 balance = newBalance;
);
The effect of this scenario is that Task 1 did nothing, because in
Step 4 Task 2 overrides Task 1's result. Obviously, the problem is
that Task 1 and Task 2 are accessing a common resource in a way
that causes conflict. This is a common problem known as a race
condition in multithreaded programs. A class is said to be thread-
safe if an object of the class does not cause a race condition in the
presence of multiple threads. As demonstrated in the preceding
example, the Account class is not thread-safe.
14
The synchronized keyword
To avoid race conditions, more than one thread must be prevented
from simultaneously entering certain part of the program, known as
critical region. The critical region in the Listing 29.7 is the entire
deposit method. You can use the synchronized keyword to
synchronize the method so that only one thread can access the
method at a time. There are several ways to correct the problem in
Listing 29.7, one approach is to make Account thread-safe by adding
the synchronized keyword in the deposit method in Line 45 as
follows:
public synchronized void deposit(double amount)
15
Synchronizing Instance Methods and Static
Methods
A synchronized method acquires a lock before it executes.
In the case of an instance method, the lock is on the object
for which the method was invoked. In the case of a static
method, the lock is on the class. If one thread invokes a
synchronized instance method (respectively, static method)
on an object, the lock of that object (respectively, class) is
acquired first, then the method is executed, and finally the
lock is released. Another thread invoking the same method
of that object (respectively, class) is blocked until the lock is
released.
16
Synchronizing Instance Methods and Static
Methods
With the deposit method synchronized, the preceding scenario
cannot happen. If Task 2 starts to enter the method, and Task 1 is
already in the method, Task 2 is blocked until Task 1 finishes the
method.
Task 1 Task 2
Acquire a-char
locktoken
on the object account -char token
+getToken +getToken
-char token +setToken +setToken
+paintComponet +paintComponet
Execute
+getToken the deposit method
+mouseClicked +mouseClicked
+setToken
+paintComponet Wait to acquire the lock
-char token
+mouseClicked
-char token
+getToken Release the lock
+setToken
+getToken
+paintComponet
-char token Acqurie a lock on the object account
+setToken
+mouseClicked
+paintComponet
+getToken -char token
+mouseClicked
+setToken
+paintComponet Execute the deposit method
+getToken
+mouseClicked +setToken
+paintComponet
-char token
+mouseClicked
+getToken Release the lock
+setToken
+paintComponet
17
Synchronizing Statements
Invoking a synchronized instance method of an object acquires a lock
on the object, and invoking a synchronized static method of a class
acquires a lock on the class. A synchronized statement can be used to
acquire a lock on any object, not just this object, when executing a
block of the code in a method. This block is referred to as a
synchronized block. The general form of a synchronized statement is
as follows:
synchronized (expr) {
statements;
}
The expression expr must evaluate to an object reference. If the
object is already locked by another thread, the thread is blocked until
the lock is released. When a lock is obtained on the object, the
statements in the synchronized block are executed, and then the lock
is released.
18
Synchronizing Statements vs. Methods
Any synchronized instance method can be converted into a
synchronized statement. Suppose that the following is a synchronized
instance method:
public synchronized void xMethod() {
// method body
}
This method is equivalent to
public void xMethod() {
synchronized (this) {
// method body
}
}
19
wait(), notify(), and notifyAll()
Use the wait(), notify(), and notifyAll() methods to facilitate
communication among threads.
The wait(), notify(), and notifyAll() methods must be called in a
synchronized method or a synchronized block on the calling object of
these methods. Otherwise, an IllegalMonitorStateException would
occur.
The wait() method lets the thread wait until some condition occurs.
When it occurs, you can use the notify() or notifyAll() methods to
notify the waiting threads to resume normal execution. The
notifyAll() method wakes up all waiting threads, while notify() picks
up only one thread from a waiting queue.
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