Please clarify my queries regarding Singleton and Multithreading:
getInstance()
method at the same time?getInstance()
synchronized
?Yes, it is necessary. There are several methods you can use to achieve thread safety with lazy initialization:
Draconian synchronization:
private static YourObject instance;
public static synchronized YourObject getInstance() {
if (instance == null) {
instance = new YourObject();
}
return instance;
}
This solution requires that every thread be synchronized when in reality only the first few need to be.
private static final Object lock = new Object();
private static volatile YourObject instance;
public static YourObject getInstance() {
YourObject r = instance;
if (r == null) {
synchronized (lock) { // While we were waiting for the lock, another
r = instance; // thread may have instantiated the object.
if (r == null) {
r = new YourObject();
instance = r;
}
}
}
return r;
}
This solution ensures that only the first few threads that try to acquire your singleton have to go through the process of acquiring the lock.
private static class InstanceHolder {
private static final YourObject instance = new YourObject();
}
public static YourObject getInstance() {
return InstanceHolder.instance;
}
This solution takes advantage of the Java memory model's guarantees about class initialization to ensure thread safety. Each class can only be loaded once, and it will only be loaded when it is needed. That means that the first time getInstance
is called, InstanceHolder
will be loaded and instance
will be created, and since this is controlled by ClassLoader
s, no additional synchronization is necessary.