Матеріали курсу ECS518U - Operating Systems - 2025/26 та відповіді на тести | qmplus.qmul.ac.uk

A Mailbox class is to be used by multiple threads. The intended behaviour is:

- When the mail box is written, it becomes occupied. When the contents are read, the mailbox becomes empty.

- The write method is used to place something in the mailbox. The method should block if the mailbox is already occupied.

- The read method is used to extract something from the mailbox. The method should block if the mailbox is empty.

Consider the code below, which is not thread safe. Select all statements that are true. Incorrect selections reduce your overall marks for this question.

public class Mailbox <T>

{

private T contents ;

public Mailbox() {

contents = null ;

}

public void write(T entry) {

contents = entry ;

}

public T read() {

T temp = contents;

contents = null;

return temp;

}

The method ‘read’ does not need to be synchronized, only the write method needs to be made synchronized.

❌

The ‘sleep()’ method is an alternative to ‘wait()’ as either can be used to pause a thread for a defined period of time.

In the read method, we can implement the desired functionality by adding the following control:

if (contents == null) {

wait() ;

}

In order to implement the desired functionality, the write method should include (at least) the following code:

while (contents != null) {

wait() ;

}

contents = entry ;

notifyAll() ;

✅

Since the ‘Mailbox’ class must contain (at least) a synchronized write method to make it thread safe, Mailbox should extend the Thread class (or implement the ‘Runnable’ interface).

The ‘wait’ method can only be called from inside a synchronized method/block.

✅

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ECS518U - Operating Systems - 2025/26

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