1. 概述
Simply put, a lock is a more flexible and sophisticated thread synchronization mechanism than the standard synchronized block.
The Lock interface has been around since Java 1.5. It's defined inside the java.util.concurrent.lock package and it provides extensive operations for locking.
In this article, we'll explore different implementations of the Lock interface and their applications.
2. Lock 与 Synchronized 的区别
There are few differences between the use of synchronized block and using Lock API's:
- A synchronized block is fully contained within a method – we can have Lock API's lock() and unlock() operation in separate methods
- A s_ynchronized block_ doesn't support the fairness, any thread can acquire the lock once released, no preference can be specified. We can achieve fairness within the Lock APIs by specifying the fairness property. It makes sure that longest waiting thread is given access to the lock
- A thread gets blocked if it can't get an access to the synchronized block. The Lock API provides tryLock() method. The thread acquires lock only if it's available and not held by any other thread. This reduces blocking time of thread waiting for the lock
- A thread which is in “waiting” state to acquire the access to synchronized block, can't be interrupted. The Lock API provides a method lockInterruptibly() which can be used to interrupt the thread when it's waiting for the lock
3. Lock API接口
Let's take a look at the methods in the Lock interface:
- void lock() – acquire the lock if it's available; if the lock isn't available a thread gets blocked until the lock is released
- void lockInterruptibly() – this is similar to the lock(), but it allows the blocked thread to be interrupted and resume the execution through a thrown java.lang.InterruptedException
- boolean tryLock() – this is a non-blocking version of lock() method; it attempts to acquire the lock immediately, return true if locking succeeds
- boolean tryLock(long timeout, TimeUnit timeUnit) – this is similar to tryLock(), except it waits up the given timeout before giving up trying to acquire the Lock
- void unlock() – unlocks the Lock instance
A locked instance should always be unlocked to avoid deadlock condition. A recommended code block to use the lock should contain a try/catch and finally block:
Lock lock = ...;
lock.lock();
try {
// access to the shared resource
} finally {
lock.unlock();
}
In addition to the Lock interface_,_ we have a ReadWriteLock interface which maintains a pair of locks, one for read-only operations, and one for the write operation. The read lock may be simultaneously held by multiple threads as long as there is no write.
ReadWriteLock declares methods to acquire read or write locks:
- Lock readLock() – returns the lock that's used for reading
- Lock writeLock() – returns the lock that's used for writing
4. Lock 实现类
4.1. ReentrantLock 可重入锁
ReentrantLock class implements the Lock interface. It offers the same concurrency and memory semantics, as the implicit monitor lock accessed using synchronized methods and statements, with extended capabilities.
Let's see, how we can use ReenrtantLock for synchronization:
public class SharedObject {
//...
ReentrantLock lock = new ReentrantLock();
int counter = 0;
public void perform() {
lock.lock();
try {
// Critical section here
count++;
} finally {
lock.unlock();
}
}
//...
}
We need to make sure that we are wrapping the lock() and the unlock() calls in the try-finally block to avoid the deadlock situations.
Let's see how the tryLock() works:
public void performTryLock(){
//...
boolean isLockAcquired = lock.tryLock(1, TimeUnit.SECONDS);
if(isLockAcquired) {
try {
//Critical section here
} finally {
lock.unlock();
}
}
//...
}
In this case, the thread calling tryLock(), will wait for one second and will give up waiting if the lock isn't available.
4.2. ReentrantReadWriteLock 读写锁
ReentrantReadWriteLock class implements the ReadWriteLock interface.
Let's see rules for acquiring the ReadLock or WriteLock by a thread:
- Read Lock – if no thread acquired the write lock or requested for it then multiple threads can acquire the read lock
- Write Lock – if no threads are reading or writing then only one thread can acquire the write lock
Let's see how to make use of the ReadWriteLock:
public class SynchronizedHashMapWithReadWriteLock {
Map<String,String> syncHashMap = new HashMap<>();
ReadWriteLock lock = new ReentrantReadWriteLock();
// ...
Lock writeLock = lock.writeLock();
public void put(String key, String value) {
try {
writeLock.lock();
syncHashMap.put(key, value);
} finally {
writeLock.unlock();
}
}
...
public String remove(String key){
try {
writeLock.lock();
return syncHashMap.remove(key);
} finally {
writeLock.unlock();
}
}
//...
}
For both the write methods, we need to surround the critical section with the write lock, only one thread can get access to it:
Lock readLock = lock.readLock();
//...
public String get(String key){
try {
readLock.lock();
return syncHashMap.get(key);
} finally {
readLock.unlock();
}
}
public boolean containsKey(String key) {
try {
readLock.lock();
return syncHashMap.containsKey(key);
} finally {
readLock.unlock();
}
}
For both read methods, we need to surround the critical section with the read lock. Multiple threads can get access to this section if no write operation is in progress.
4.3. StampedLock 邮戳锁
StampedLock is introduced in Java 8. It also supports both read and write locks. However, lock acquisition methods return a stamp that is used to release a lock or to check if the lock is still valid:
public class StampedLockDemo {
Map<String,String> map = new HashMap<>();
private StampedLock lock = new StampedLock();
public void put(String key, String value){
long stamp = lock.writeLock();
try {
map.put(key, value);
} finally {
lock.unlockWrite(stamp);
}
}
public String get(String key) throws InterruptedException {
long stamp = lock.readLock();
try {
return map.get(key);
} finally {
lock.unlockRead(stamp);
}
}
}
Another feature provided by StampedLock is optimistic locking. Most of the time read operations don't need to wait for write operation completion and as a result of this, the full-fledged read lock isn't required.
Instead, we can upgrade to read lock:
public String readWithOptimisticLock(String key) {
long stamp = lock.tryOptimisticRead();
String value = map.get(key);
if(!lock.validate(stamp)) {
stamp = lock.readLock();
try {
return map.get(key);
} finally {
lock.unlock(stamp);
}
}
return value;
}
5. 使用 Conditions
The Condition class provides the ability for a thread to wait for some condition to occur while executing the critical section.
This can occur when a thread acquires the access to the critical section but doesn't have the necessary condition to perform its operation. For example, a reader thread can get access to the lock of a shared queue, which still doesn't have any data to consume.
Traditionally Java provides wait(), notify() and notifyAll() methods for thread intercommunication. Conditions have similar mechanisms, but in addition, we can specify multiple conditions:
public class ReentrantLockWithCondition {
Stack<String> stack = new Stack<>();
int CAPACITY = 5;
ReentrantLock lock = new ReentrantLock();
Condition stackEmptyCondition = lock.newCondition();
Condition stackFullCondition = lock.newCondition();
public void pushToStack(String item){
try {
lock.lock();
while(stack.size() == CAPACITY) {
stackFullCondition.await();
}
stack.push(item);
stackEmptyCondition.signalAll();
} finally {
lock.unlock();
}
}
public String popFromStack() {
try {
lock.lock();
while(stack.size() == 0) {
stackEmptyCondition.await();
}
return stack.pop();
} finally {
stackFullCondition.signalAll();
lock.unlock();
}
}
}
6. 总结
In this article, we have seen different implementations of the Lock interface and the newly introduced StampedLock class. We also explored how we can make use of the Condition class to work with multiple conditions.
The complete code for this tutorial is available over on GitHub.