Java Thread Synchronization

●

Creating a Java Thread

●

Synchronized Keyword

●

Wait and Notify

●

●

●

Java threads are a way to have different parts of your program
running at the same time.

For example, you can create an application that accepts input
from different users at the same time, each user handled by a
thread.
Also, most networking applications involve threads
–

you would need to create a thread that would wait for incoming
messages while the rest of your program handles your outgoing
messages.

●

There are two ways to create a java thread
–

Extending the Thread class

–

Implementing the runnable interface

●

We will create a thread that simply prints out a number 500
times in a row.
class MyThread extends Thread {
int i;
MyThread(int i) {
this.i = i;
}
public void run() {
for (int ctr=0; ctr < 500; ctr++) {
System.out.print(i);
}
}
}

●

To show the difference between parallel and non-parallel
execution, we have the following executable MyThreadDemo
class
class MyThreadDemo {
public static void main(String args[]) {
MyThread t1 = new MyThread(1);
t1.run();
t2.run();
t3.run();
System.out.print("Main ends");
}}

●

Upon executing MyThreadDemo, we get output that looks like
this
$ java MyThreadDemo
1111111111...22222222.....33333......Main ends

●

●

●

As can be seen, our program first executed t1's run method,
which prints 500 consecutive 1's.
After that t2's run method, which prints consecutuve 2's, and
so on.
Main ends appears at the last part of the output as it is the last
part of the program.

●

What happened was serial execution, no multithreaded
execution occurred
–

This is because we simply called MyThread's run() method

●

To start parallel execution, we call MyThread's start() method,
which is built-in in all thread objects.
t1.start();
t2.start();
t3.start();
}}

●

When we run MyThreadDemo we can definitely see that three
run methods are executing at the same time
> java MyThreadDemo
111111111122331122321122333331111Main ends111333222...

●

Note the appearance of the "Main ends" string in the middle of
the output sequence
> java MyThreadDemo
111111111122331122321122333331111Main ends111333222...

●

This indicates that the main method has already finished
executing while thread 1, thread 2 and thread 3 are still
running.

●

Running a main method creates a thread.
–

●

Normally, your program ends when the main thread ends.

However, creating and then running a thread's start method
creates a whole new thread that executes its run() method
independent of the main method.

●

●

Another way to create a thread is to implement the Runnable
interface.

This may be desired if you want your thread to extend another
class.
–

By extending another class, you will be unable to extend class thread
as Java classes can only extend a single class.

–

However, a class can implement more than one interface.

●

The following is our MyThread class created by implementing
the runnable interface.
class MyThread implements Runnable {
... <thread body is mostly the same>
}
●

Classes that implement Runnable are instantiated in a
different way.
–

Thread t1 = new Thread(new MyThread(1));

●

●

Note that implementing an interface requires you to define all
the methods in the interface

For the Runnable interface, you are required in your
implementing class to define the run() method or your
program will not run.

………………

……………………………
Pausing and Joining
Threads

Threads can be paused by the sleep() method.

●

For example, to have MyThread pause for half a second
before it prints the next number, we add the following lines of
code to our for loop.

●

try {
Thread.sleep(500); // 500 miliseconds
} catch (InterruptedException e) { }
}

●

Thread.sleep() is a static method of class thread and can
be invoked from any thread, including the main thread.

try {
Thread.sleep(500); // 500 miliseconds
}

●

The InterruptedException is an unchecked exeception that is
thrown by code that stops a thread from running.
–

●

Unchecked exception causing lines must be enclosed in a try-catch, in
this case, Thread.sleep().

An InterruptedException is thrown when a thread is waiting,
sleeping, or otherwise paused for a long time and another
thread interrupts it using the interrupt() method in class
Thread.

●

●

You can also make a thread stop until another thread finishes
running by invoking a thread's join() method.

For instance, to make our main thread to stop running until
thread t1 is finished, we could simply say...
...
t1.start();
try {
t1.join();
t2.start();
●

This would cause all the 1's to be printed out before thread
2 and thread 3 start.

………………

……………………………
Critical Section
Problem

●

●

This section we will find out how to implement a solution to the
critical section problem using Java

Recall that only a single process can enter its critical section,
all other processes would have to wait
–

No context switching occurs inside a critical section

●

Instead of printing a continuous stream of numbers, MyThread
calls a print10() method in a class MyPrinter
–

●

print10() prints 10 continuous numbers on a single line before a
newline.

Our goal is to have these 10 continuous numbers printed
without any context switches occuring.
–

Our output should be:
...
1111111111
1111111111
2222222222
3333333333
1111111111
...

●

We define a class MyPrinter that will have our print10()
method

class MyPrinter {
public void print10(int value) {
for (int i = 0; i < 10; i++) {
System.out.print(value);
}
System.out.println(""); // newline after 10 numbers
}
}

●

Instead of printing numbers directly, we use the print10()
method in MyThread, as shown by the following
int i;
MyPrinter p;
MyThread(int i) {
this.i = i;
p = new MyPrinter();
}
public void run() {
p.print10(i);
}
}
}

●

First, we will try to see the output of just a single thread
running.
// MyThread t2 = new MyThread(2);
// MyThread t3 = new MyThread(3);
t1.start();
// t2.start();
// t3.start();
}}

We comment out
the other threads
for now...

●

When we run our MyThreadDemo, we can see that the output
really does match what we want.
> java MyThreadDemo
1111111111
1111111111
1111111111
1111111111
1111111111
...

●

However, let us try to see the output with other threads.

t1.start();
t2.start();
t3.start();
}}

We run all three
threads

Our output would look like the following

●

> java MyThreadDemo
1111111111
111112222222
1111
22233333332
...
●

We do not achieve our goal of printing 10 consecutive
numbers in a row
–

all three threads are executing the print10's method at the same
time, thus having more than one number appearing on a single
line.

●

To achieve our goal there should not be any context switches
happening inside print10()
–

●

●

Only a single thread should be allowed to execute inside print10()

As can be seen, this is an example of the critical section
problem
To solve this, we can use some of the techniques discussed in
our synchronization chapter

●
●
●
●

Busy Wait
Wait and Notify

Semaphores
Monitors

●

●

Now we will discuss Java's
solution to the critical
section problem
Java uses a monitor
construct
–

Only a single thread may run
inside the monitor

Even if P2 is calling method2(), it
has to wait for P1 to finish

●

To turn an object into a monitor, simply put the synchronized
keyword on your method definitions
–

Only a single thread can run any synchronized method in an object

class MyPrinter {
public synchronized void print10(int value) {
for (int i = 0; i < 10; i++) {
}
System.out.println(""); // newline after 10
numbers
}
}

However, even if we did turn our print10() method into a
synchronized method, it will still not work

●

> java MyThreadDemo
1111111111
111112222222
1111
22233333332
...

●

To find out how to make it work, we need to first find out
how the synchronized keyword works

●

Every object in Java has an intrinsic lock

●

When a thread tries to run a synchronized method, it first tries
to get a lock on the object

●

If a thread is successful, then it owns the lock and executes
the synchronized code.

●

Other threads cannot run the synchronized code because the
lock is unavailable

●

Threads can only enter once the thread owning the lock
leaves the synchronized method

●

Waiting threads would then compete on who gets to go next

●

So why doesn't our example work?

●

Because each thread has its own copy of the MyPrinter object!

int i;
MyPrinter p;
MyThread(int i) {
this.i = i;
p = new MyPrinter();
// each MyThread creates
its own MyPrinter!
}
public void run() {
p.print10(i);
}
}
}

●

Recall our definition of synchronized methods
–

●

Only a single thread can run any synchronized method in an object

Since each thread has its own MyPrinter object, then no
locking occurs

●

So, to make it work, all threads must point to the same
MyPrinter object

●

Note how all MyThreads now have the same MyPrinter object

int i;
MyPrinter p;
MyThread(int i, MyPrinter p) {
this.i = i; this.p = p
}
public synchronized void run() {
for (int ctr=0; ctr < 500;
ctr++) {
p.print10(i);
}
}
}

MyPrinter p = new
MyPrinter();
MyThread t1 = new
MyThread(1,p);
MyThread t2 = new
MyThread(2,p);
MyThread t3 = new
MyThread(3,p);
t1.start();
t2.start();
t3.start();
}}

●

A way to visualize it is that all Java objects can be doors
–

A thread tries to see if the door is open

–

Once the thread goes through the door, it locks it behind it,

–

No other threads can enter the door because our first thread has
locked it from the inside

–

Other threads can enter if the thread inside unlocks the door and goes
out

–

Lining up will only occur if everyone only has a single door to the
critical region

●

Running MyThreadDemo now correctly shows synchronized
methods at work
>java MyThreadDemo
1111111111
1111111111
1111111111
2222222222
3333333333
...

●

Only a single thread can run any synchronized method in an
object
–

●

If an object has a synchronized method and a regular method, only one
thread can run the synchronized method while multiple threads can run
the regular method

Threads that you want to have synchronized must share the
same monitor object

●

●

Aside from synchronized methods, Java also allows for
synchronized blocks.

You must specify what object the intrinsic lock comes from

●

Our print10() implementation, done using synchronized
statements, would look like this

class MyPrinter {
synchronized(this) {
for (int i = 0; i < 10; i++) {
}
System.out.println(""); // newline after
10 numbers
}
}
}
We use a MyPrinter object for the
lock, replicating what the
synchronized method does

●

●

Synchronized blocks allow for flexibility, in the case if we want
our intrinsic lock to come from an object other than the current
object.
For example, we could define MyThread's run method to
perform the lock on the MyPrinter object before calling print10

MyPrinter p;
...
public void run() {
for (int i = 0; i < 100; i++) {
synchronized(p) {
p.print10(value);
}
}}}

Note that any lines
before and after our
synchronized block can
be executed
concurrently by threads

●

Also, the use of the synchronized block allows for more
flexibility by allowing different parts of code to be locked with
different objects.

●

For example, consider a modified MyPrinter which has two
methods, which we'll allow to run concurrently

class MyPrinter {
The blocks inside print10() and
Object lock1 = new Object();
squareMe() can run at the same time
Object lock2 = new Object();
because they use different objects for
their locking mechanism
synchronized(lock1) {
for (int i = 0; i < 10; i++) {
}
System.out.println(""); // newline after 10 numbers
}
}
Synchronization still applies. For
public int squareMe(int i) {
example only a single thread can
synchronized (lock2) {
run the synchronized block in
return i * i;
squareMe() at any point in time
}
}
}

●

Creating a Java Thread

●

Synchronized Keyword

●

Wait and Notify

●

High Level Cuncurrency Objects

●

●

We will now discuss a solution to the Producer-Consumer
problem.

Instead of using beer, the producer will store in a shared stack
a random integer value which will be retrieved by the
consumer process.

●

As can be seen, we would need to have a place to store our
array of integers and our top variable, which should be shared
among our producer and consumer.
–

MAXCOUNT is the maximum number of items the Producer produces

–

We'll assume the maximum array size of 10000 for now

class SharedVars {
final int MAXCOUNT = 100;
int array[] = new int[10000];
int top;
// <more code to follow>
}

●

●

To keep things object-oriented, we will place the code for
placing values in the stack and getting values in the stack in
our SharedVars class
Our next slide shows our implementation

class SharedVars {
int top;
// method for inserting a value
public void insertValue(int value) {
if (top < array.length) {
array[top] = value;
top++;
}
}
// method for getting a value
public int getValue() {
if (top > 0) {
top--;
return array[top];
}
else
return -1;

}}

●

●

Now we will slowly build our Producer and Consumer class
We can say that our Producer and Consumer should be
implemented as threads.
–

Thus we have our code below, including a class that starts these
threads

class Producer extends Thread {
public void run() {
}
}

class Consumer extends Thread {
public void run() {
}
}

class PCDemo {
Producer p = new Producer();
Consumer c = new Consumer();
p.start();
c.start();
}}

●

Producer and Consumer should have the same SharedVars
object
SharedVars sv;
Producer(SharedVars sv) {
this.sv = sv;
}
public void run() {
}
}

SharedVars sv;
Consumer(SharedVars sv) {
this.sv = sv;
}
public void run() {
}
}

class PCDemo {
SharedVars sv = new
SharedVars();
Producer p = new Producer(sv);
Consumer c = new Consumer(sv);
p.start();
c.start();
}}

●

There is no more need to modify our PCDemo class so we will
not show it anymore.

●

We want our producer to produce a random integer and store
it in SharedVars. Thus our Producer code would look like this
SharedVars sv;
this.sv = sv;
}
public void run() {
int value = (int)(Math.random() * 100); // random value from
0 to 100
sv.insertValue(value);
System.out.println("Producer: Placing value: " + value);
}
}

●

We want our Producer to do this a hundred times, so we will
add a loop to our code
SharedVars sv;
this.sv = sv;
}
public void run() {
for (int i = 0; i < 100; i++) {
int value = (int)(Math.random() * 100); // random
value from 0 to 100
System.out.println("Producer: Placing value: " +
value);
}
}
}

●

Finally, we have our producer pause for a maximum of 5
seconds each time it places a value
SharedVars sv;
this.sv = sv;
}
public void run() {
for (int i = 0; i < 100; i++) {
int value = (int)(Math.random() * 100); // random
value from 0 to 100
System.out.println("Producer: Placing value: " +
value);
try {
Thread.sleep((int)(Math.random() *
5000)); // sleep 5s max

}
}

}

●

Now, we want our consumer to get a value from SharedVars

SharedVars sv;
this.sv = sv;
}
public void run() {
int value = sv.getValue();
System.out.println("Consumer: I got value:" + value);
}
}

●

We want to get a value 100 times

SharedVars sv;
this.sv = sv;
}
public void run() {
for (int i = 0; i < 100; i++) {
System.out.println("Consumer: I got value:" +
value);
}
}
}

●

Finally, just like producer, we pause for a maximum of 5
seconds on each loop
SharedVars sv;
this.sv = sv;
}
public void run() {
for (int i = 0; i < 100; i++) {
System.out.println("Consumer: I got value:" +
value);

try {
Thread.sleep((int)(Math.random() *
5000)); // sleep 5s max

}
}
}

●

As was discussed in a previous chapter, we try to avoid a race
condition between insertValue() and getValue()

Consumer getValue(): top = top – 1
Producer insertValue(): array[top] = value // this would overwrite an existing value!
Producer insertValue(): top = top + 1
Consumer getValue(): return array[top] // consumer returns an already returned value

●

●

We do not want getValue() and insertValue() to run at the
same time

Therefore, we modify SharedVars to use synchronized blocks
–

We could also use synchronized methods but we need some nonsynchronized commands

class SharedVars {
int top;
public void insertValue(int value) { // method for inserting a value
array[top] = value;
top++;
}
}
}
public int getValue() { // method for getting a value
if (top > 0) {
top--;
return array[top];
}
else
return -1;
}
}
}

Upon executing our program, the output of our code would
look something like this:

●

Producer inserts value: 15
Consumer got: 15
Consumer got: -1
Consumer got: 75
...
●

Note that both Consumer and Producer are running at the
same time.

Consumer got: 15
Consumer got: -1
Consumer got: 75
...
●

Notice that, if we try to get a value from the array and there
isn't any, we return a value -1.
–

–
●

You can see this clearly if you increase the Producer delay to 10s.

Producer adds a value every 10s, consumer gets one every 5s

Wouldn't it be better if we have our Consumer wait for the
Producer to produce something instead of just getting -1?

We could implement a busy wait for this

●

while (top <= 0) { } // do nothing
top--;
return array[top];
}}
●

Note how we placed this outside our synchronized block
–

Having our busy wait inside would mean blocking out producer
thread from insertValue(), which is what is needed to break the
busy wait

●

●

●

A better solution is to use the wait() method, defined in class
Object, and is therefore inherited by all objects

A thread invoking wait() will suspend the thread
However, a thread invoking wait() must own the intrinsic lock
of the object it is calling wait() from
–

●

If we are going to call this.wait() it has to be in synchronized(this) block

Also, as with all methods that suspend thread execution, like
join() and sleep(), our wait() method must be in a try-catch
block that catches InterruptedExceptions.

// called by Consumer thread
if (top <= 0) {
try {
this.wait();
}
top--;
return array[top];

}}

●

●

All threads that call wait() on an object are placed in a pool of
waiting threads for that object.

Execution resumes when another thread calls the notify()
method of the object our first thread is waiting on
–

The notify() method is defined in class Object.

–

All objects have wait() and notify()

●

For our example, our producer thread, after inserting on an
empty array, would notify the consumer thread that it has
placed a value in our array by calling the notify() method on
the SharedVars object
–

Recall that Producer and Consumer both have a reference to the same
SharedVars object.

–

The producer calling notify() from insertValue() would inform any the
consumer waiting on the same SharedVar object.

// called by producer
array[top] = value;
top++;
if (top == 1) { // we just inserted on an empty array
this.notify(); // notify sleeping thread
}
}
}
}

class SharedVars {
int top;
array[top] = value;
top++;
if (top == 1) { // we just inserted on an empty array
this.notify(); // notify any sleeping thread
}
}
}
}
if (top <= 0) {
try {
this.wait();
}
top--;
return array[top];
}
}
}

●

●

When the notify() method of an object is called, then a single
waiting thread on that object is signaled to get ready to
resume execution.
After our Producer thread exits insertValue and releases the
lock, our Consumer thread gets the lock once again and
resumes its execution.

●

●

Another method called notifyAll() notifies all the waiting
threads.

These waiting threads would then compete to see which
single thread resumes execution, the rest of the threads would
once again wait.

●

●

If you run the demo program again, you can see that, even if
the producer is slow, the consumer waits until the producer
gives out a value before getting that value
Note that you must also consider inserting on a full array
–

Producer must wait until consumer has taken away some values
before inserting

–

We will leave the implementation of this as an exercise

●

Note that this same thing must occur on inserting on a full
array
–

Producer must wait until consumer has taken away some values
before inserting

–

We will leave the implementation of this as an exercise.

●

●

Alice and Bob are at the office. There is a doorway that they have
to pass through every now and then. Since Bob has good manners,
if he gets to the door, he always makes sure that he always opens
the door and waits for Alice to go through before he does. Alice by
herself simply goes through the door.
Your task is to write an Alice and Bob thread that mimics this
behavior. To simulate having to pass through the door, have a delay
in each thread for a random amount of time, maximum of 5
seconds. The output of your code should look something like this:
Alice: I go through the door

Bob: I get to the door and wait for Alice...

Bob: I follow Alice
Bob: I get to the door and wait for Alice

Java Thread Synchronization

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Java Thread Synchronization (20)

Recently uploaded (20)

Java Thread Synchronization