1. Locking in Java

2. The ReentrantLock class
ReentrantLock lck=new ReentrantLock();
void withdraw(long amt) throws Exception {
lck.lock();
try
{ do stuff… }
finally { lck.unlock(); } }
void deposit(long amt)
balance=balance+amt;
lck.unlock();
Why the ‘finally’ block for withdraw and not deposit?

3. Java’s synchronized statement
void withdraw(long amt) throws Exception {
synchronized(this)
{ do stuff… } }
void deposit(long amt)
{ balance=balance+amt; }
The block of code covered by ‘synchronized’ will lock on the object passed in; almost always ‘this’ for convenience
With ‘synchronized’ we are guaranteed that leaving that block of code will release the lock, even if the code throws an exception or returns