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.
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; } }
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; } }