How can we ensure that instances of this class can be safely used by multiple threads?-CodePudding

This class is intended to allow users to write a series of messages, so that each message is identified with a timestamp and the name of the thread that wrote the message

public class Logger {
    private StringBuilder contents = new StringBuilder();

    public void log(String message) {
        contents.append(System.currentTimeMillis());
        contents.append(": ");
        contents.append(Thread.currentThread().getName());
        contents.append(message);
        contents.append("\n");
    }

    public String getContents() {
        return contents.toString();
    }
}

CodePudding user response：

If you really intend to collect logs like this, the most simple approach would be to use a synchronized section to make sure that only one thread at a time can access the contents field:

public class Logger {

    private final StringBuilder contents = new StringBuilder();

    public void log(String message) {
        synchronized (contents) {
            contents.append(System.currentTimeMillis());
            contents.append('[');
            contents.append(Thread.currentThread().getName());
            contents.append("]: ");
            contents.append(message);
            contents.append("\n");
        }
    }

    public String getContents() {
        synchronized (contents) {
            return contents.toString();
        }
    }
}

The field should be final in order to safely serve as a lock.

More about synchronized: https://www.baeldung.com/java-synchronized#the-synchronized-keyword

CodePudding user response：

You can constrain exclusive access to a Logger object by simply adding the synchronized keyword to each method.

public class Logger {

    private StringBuilder contents = new StringBuilder();

    public synchronized void log(String message) {
        contents.append(System.currentTimeMillis());
        contents.append('[');
        contents.append(Thread.currentThread().getName());
        contents.append("]: ");
        contents.append(message);
        contents.append("\n");
    }

    public synchronized String getContents() {
        return contents.toString();
    }
}

This is equivalent to

public class Logger {

    private final StringBuilder contents = new StringBuilder();

    public void log(String message) {
        synchronized (this) {
            contents.append(System.currentTimeMillis());
            contents.append('[');
            contents.append(Thread.currentThread().getName());
            contents.append("]: ");
            contents.append(message);
            contents.append("\n");
        }
    }

    public String getContents() {
        synchronized (this) {
            return contents.toString();
        }
    }
}