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example.java
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318 lines (228 loc) · 6.06 KB
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//this code will give errors and unpredictable behaviour
public class Main {
public static void main(String[] args) {
List<Object> list = new LinkedList<>();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 20; i++) {
list.add(1);
}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
});
t1.start();
t2.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print(list);
}
}
//to fix this we can use Collections.synchronizedList(line 52) to wrap the list we are using
public class Main {
public static void main(String[] args) {
//List<Object> list = new LinkedList<>();
List<Object> list = Collections.synchronizedList(new LinkedList<>());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 20; i++) {
list.add(1);
}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
});
t1.start();
t2.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print(list);
}
}
//output:
[1, 1, 2, 1, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 1, 2, 2, 2]
//however the order in which elements are set is still unpredictable,
//to fix this we need to use synchronized blocks
public class Main {
public static void main(String[] args) {
//List<Object> list = new LinkedList<>();
List<Object> list = Collections.synchronizedList(new LinkedList<>());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(1);
}
}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
}
});
t1.start();
t2.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print(list);
}
}
//output:
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2]
//and if we comment the first synchronized block t1 will add its numbers until t2 starts its synchronized block
//at which point t2 acquires the lock over the list and t1 has to wait until t2 adds all of its numbers
public class Main {
public static void main(String[] args) {
//List<Object> list = new LinkedList<>();
List<Object> list = Collections.synchronizedList(new LinkedList<>());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
//synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(1);
}
//}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
}
});
t1.start();
t2.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.print(list);
}
}
//output:
[1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
//or
[1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
//or similar where all 2's are combined
//this behavior may be desired in some circumstances
//you may think that putting System.out.print(line 193) into the synchronized block would be a good idea
//because there too the list is accessed and you would be correct
public class Main {
public static void main(String[] args) {
//List<Object> list = new LinkedList<>();
List<Object> list = Collections.synchronizedList(new LinkedList<>());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
//synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(1);
}
//}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
}
});
t1.start();
t2.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (list) {
System.out.print(list);
}
}
}
//output:
[1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
//or
[1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
//or similar where all 2's are combined
//that leaves only one issue, what happens if we remove Thread.sleep(1000);(line 188)
//answer is that there will be no error however only part of the list that was filled up to that point will be printed
public class Main {
public static void main(String[] args) {
//List<Object> list = new LinkedList<>();
List<Object> list = Collections.synchronizedList(new LinkedList<>());
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
//synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(1);
}
//}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (list) {
for (int i = 0; i < 20; i++) {
list.add(2);
}
}
}
});
t1.start();
t2.start();
//try {
// Thread.sleep(1000);
//} catch (InterruptedException e) {
// e.printStackTrace();
//}
synchronized (list) {
System.out.print(list);
}
}
}
//output:
[]
//this is happening bacause synchronized blocks of t2 and main threads are raceing to get access to the list
//and most often main thread is the winner so list is printed before any elements are added