1. 1 Where did the synchronized methods go? Yes, there still exists the synchronized keyword. public synchronized void foo() {} public void foo(){ aLock.lock(); // atomic code aLock.unlock(); } The scope of a lock is always “per-method” in a synchronized method. You can define arbitrary locking scope with ReentrantLock.

2. 2 Concurrency: Double-edged Sword Must protect shared resources –Identify potential race conditions What must be protected? Who access shared resources? When shared resources are accessed/changed? –Synchronizing threads Why not have all methods use locks? The highest exec overhead in Java program –Lock acquisition/release –Thread synchronization –Garbage collection Need to minimize the use of locking and thread synchronization

3. 3 Minimizing Threading Errors A preventive good practice/habit –Focus on the immutability of objects Immutable objects –Objects that never change its state. Only getter methods; no setter methods. No need to worry about race conditions. –e.g. String (java.lang.String) char str[] = {’u', ’m', ’b'}; String str = new String(str); String str = “umb”; –A series of constructors to initialize a string data. –All other methods never change the content of the initialized string data.

4. 4StringTest.java Output –umb –3 –umb –uml –umb –UMB –umb

5. 5 String Class The initialized string data (“umb”) is maintained in a private variable in String. –To prevent external objects from updating it. The private variable is also final. –Once a value is assigned to a final variable, the value cannot be changed afterward. Two ways for value assignment –private final String data = “uml”; –Use a constructor. –To prevent methods from updating the initialized string data (“umb”).

6. 6 String is a final class. –public final class String extends Object implements Serializable, Comparable, CharSequence –A final class cannot be extended to define subclasses. –To prevent sub classes from updating the initialized string data. See StringTest2.java

7. 7 Fragile Immutable Class The StrData is immutable itself, but it is fragile. –It becomes mutable once its subclass defines a setter method. –Mutable A setter method(s) exists. Thread unsafe if no thread synchronization is implemented. Remember ThreadSafeBankAccount. –public class ThreadSafeBankAccount{ private double balance = 0; … }

8. 8 You Need to… Whenever appropriate, use final and private as often as possible in threaded programs. –A small changes can cause race conditions. –Have your compiler work harder! Clearly state immutability in program comments, API documents, design documents, etc. –Use {frozen} in UML class diagrams

9. 9 Other Immutable Classes Wrapper classes for primitive types –java.lang.Boolean for boolean –java.lang.Byte for byte –java.lang.Character for char –java.lang.Double for double –java.lang.Float for floar –java.lang.Integer for int –java.lang.Long for long –java.lang.Short for short –…etc.

10. 10 Common Pitfalls Read Mutable.java and Mutable2.java Mutable and Mutable2 are mutable –although they have no setter methods.

11. 11 Note that a variable contains a pointer (reference) to an object –NOT the object itself. –When typed as primitive type, a variable contains a primitive value itself. No pointers (references) to primitive values.

12. 12 Minimizing Performance Loss Separate (read and write) locks when you have reader and writer threads –Readers do not have to compete for a shared resource. Use an immutable object when you have reader threads only. Use the “volatile” keyword when you have writer threads only. Use thread-specific storage.

13. 13 Performance Implication of Immutable Objects Immutable objects are good for performance as well as threading error avoidance. –Unnecessary lock acquisition/release and thread synchronization. However, in practice, most objects have both getter and setter methods… Think of separating your object to mutable and immutable objects –if the object is read very often.

14. 14 An Example: String and StringBuffer Both represent a String data. String –immutable. –String’s methods do not use locking. StringBuffer –mutable. –StringBuffer’s methods use locking to read/write a String data. String is faster to read a String data. –Note: StringBuffer is faster to create a String data from multiple String data.