Double-checked locking can be used for lazy initialization of volatile fields, but only if field assignment is the last step in the
synchronized block. Otherwise you run the risk of threads accessing a half-initialized object.
Noncompliant code example
public class MyClass {
private volatile List<String> strings;
public List<String> getStrings() {
if (strings == null) { // check#1
synchronized(MyClass.class) {
if (strings == null) {
strings = new ArrayList<>(); // Noncompliant
strings.add("Hello"); //When threadA gets here, threadB can skip the synchronized block because check#1 is false
strings.add("World");
}
}
}
return strings;
}
}
Compliant solution
public class MyClass {
private volatile List<String> strings;
public List<String> getStrings() {
if (strings == null) { // check#1
synchronized(MyClass.class) {
if (strings == null) {
List<String> tmpList = new ArrayList<>();
tmpList.add("Hello");
tmpList.add("World");
strings = tmpList;
}
}
}
return strings;
}
}
