As I understand, I am suppose to call wait() on the mutex, when I want the current thread to stop working until another thread calls notify() on the same mutex object. That doesn't seem to be working.
I'm trying to make a thread print 1-10. Then wait for another thread to print 11-20. And then the first thread would again print 21-30
Main.java
public class Main {
public static void main(String[] args) throws InterruptedException {
Object mutex = 1;
Thread child1 = new Thread(new Child1(mutex));
Thread child2 = new Thread(new Child2(mutex));
child1.start();
child2.start();
}
}
Child1.java
public class Child1 implements Runnable {
Object mutex;
public Child1(Object mutex){
this.mutex = mutex;
}
public void run() {
synchronized (mutex) {
for(int c = 0; c < 10; c++){
System.out.println(c+1);
}
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
for(int c = 20; c < 31; c++){
System.out.println(c+1);
}
}
}
Child2.java
public class Child2 implements Runnable {
Object mutex;
public Child2(Object mutex) {
this.mutex = mutex;
}
public void run() {
synchronized (mutex) {
for (int c = 11; c < 21; c++) {
System.out.println(c);
}
notify();
}
}
}
Output
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17Exception in thread "Thread-0"
18
19
20
Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:502)
at task42.Child1.run(Child1.java:18)
at java.lang.Thread.run(Thread.java:745)
java.lang.IllegalMonitorStateException
at java.lang.Object.notify(Native Method)
at task42.Child2.run(Child2.java:15)
at java.lang.Thread.run(Thread.java:745)
What am I missing?
You must add the mutex
reference to wait()
and notify()
; that is, change wait()
to mutex.wait()
and notify()
to mutex.notify()
.
Without this, you are calling to wait/notify on this
(method()
is equivalent to this.method()
)
Here is your code with the appropriate changes made:
Child1.java
public class Child1 implements Runnable {
Object mutex;
public Child1(Object mutex){
this.mutex = mutex;
}
public void run() {
synchronized (mutex) {
for(int c = 0; c < 10; c++){
System.out.println(c+1);
}
try {
mutex.wait(); // Changed here
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
for(int c = 20; c < 31; c++){
System.out.println(c+1);
}
}
}
Child2.java
public class Child2 implements Runnable {
Object mutex;
public Child2(Object mutex) {
this.mutex = mutex;
}
public void run() {
synchronized (mutex) {
for (int c = 11; c < 21; c++) {
System.out.println(c);
}
mutex.notify(); // Changed here
}
}
}
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