一、Redisson介绍
Redisson是架设在Redis基础上的一个Java驻内存数据网格（In-Memory Data Grid）。
注意：不同的版本可能实现锁的机制并不相同
二、RLock接口
- RLock接口继承了标准接口Lock，具备标准锁接口的所有特性，比如：lock、unlock、trylock等等。
- 扩展了标准接口Lock，常用的主要有：强制锁释放，带有效期的锁，还有一组异步的方法。其中前面两个方法主要是解决标准Lock可能造成的死锁问题。比如某个线程获取到锁之后，线程所在机器死机，此时获取了锁的线程无法正常释放锁导致其余的等待锁的线程一直等待下去。
源码1：RLock
package org.redisson.api;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
/**
* Distributed implementation of {@link java.util.concurrent.locks.Lock}
* Implements reentrant lock.
* Use {@link RLock#getHoldCount()} to get a holds count.
*
* @author Nikita Koksharov
*
*/
public interface RLock extends Lock, RLockAsync {
/**
* Returns name of object
*
* @return name - name of object
*/
String getName();
/**
* Acquires the lock.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until the
* lock has been acquired.
*
* If the lock is acquired, it is held until <code>unlock</code> is invoked,
* or until leaseTime have passed
* since the lock was granted - whichever comes first.
*
* @param leaseTime the maximum time to hold the lock after granting it,
* before automatically releasing it if it hasn't already been released by invoking <code>unlock</code>.
* If leaseTime is -1, hold the lock until explicitly unlocked.
* @param unit the time unit of the {@code leaseTime} argument
* @throws InterruptedException - if the thread is interrupted before or during this method.
*/
void lockInterruptibly(long leaseTime, TimeUnit unit) throws InterruptedException;
/**
* Returns <code>true</code> as soon as the lock is acquired.
* If the lock is currently held by another thread in this or any
* other process in the distributed system this method keeps trying
* to acquire the lock for up to <code>waitTime</code> before
* giving up and returning <code>false</code>. If the lock is acquired,
* it is held until <code>unlock</code> is invoked, or until <code>leaseTime</code>
* have passed since the lock was granted - whichever comes first.
*
* @param waitTime the maximum time to aquire the lock
* @param leaseTime lease time
* @param unit time unit
* @return <code>true</code> if lock has been successfully acquired
* @throws InterruptedException - if the thread is interrupted before or during this method.
*/
boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException;
/**
* Acquires the lock.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until the
* lock has been acquired.
*
* If the lock is acquired, it is held until <code>unlock</code> is invoked,
* or until leaseTime milliseconds have passed
* since the lock was granted - whichever comes first.
*
* @param leaseTime the maximum time to hold the lock after granting it,
* before automatically releasing it if it hasn't already been released by invoking <code>unlock</code>.
* If leaseTime is -1, hold the lock until explicitly unlocked.
* @param unit the time unit of the {@code leaseTime} argument
*
*/
void lock(long leaseTime, TimeUnit unit);
/**
* Unlocks lock independently of state
*
* @return <code>true</code> if lock existed and now unlocked otherwise <code>false</code>
*/
boolean forceUnlock();
/**
* Checks if this lock locked by any thread
*
* @return <code>true</code> if locked otherwise <code>false</code>
*/
boolean isLocked();
/**
* Checks if this lock is held by the current thread
*
* @param threadId Thread ID of locking thread
* @return <code>true</code> if held by given thread
* otherwise <code>false</code>
*/
boolean isHeldByThread(long threadId);
/**
* Checks if this lock is held by the current thread
*
* @return <code>true</code> if held by current thread
* otherwise <code>false</code>
*/
boolean isHeldByCurrentThread();
/**
* Number of holds on this lock by the current thread
*
* @return holds or <code>0</code> if this lock is not held by current thread
*/
int getHoldCount();
/**
* Remaining time to live of this lock
*
* @return time in milliseconds
* -2 if the lock does not exist.
* -1 if the lock exists but has no associated expire.
*/
long remainTimeToLive();
}
源码2：Lock
package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
/**
* {@code Lock} implementations provide more extensive locking
* operations than can be obtained using {@code synchronized} methods
* and statements. They allow more flexible structuring, may have
* quite different properties, and may support multiple associated
* {@link Condition} objects.
*
* <p>A lock is a tool for controlling access to a shared resource by
* multiple threads. Commonly, a lock provides exclusive access to a
* shared resource: only one thread at a time can acquire the lock and
* all access to the shared resource requires that the lock be
* acquired first. However, some locks may allow concurrent access to
* a shared resource, such as the read lock of a {@link ReadWriteLock}.
*
* <p>The use of {@code synchronized} methods or statements provides
* access to the implicit monitor lock associated with every object, but
* forces all lock acquisition and release to occur in a block-structured way:
* when multiple locks are acquired they must be released in the opposite
* order, and all locks must be released in the same lexical scope in which
* they were acquired.
*
* <p>While the scoping mechanism for {@code synchronized} methods
* and statements makes it much easier to program with monitor locks,
* and helps avoid many common programming errors involving locks,
* there are occasions where you need to work with locks in a more
* flexible way. For example, some algorithms for traversing
* concurrently accessed data structures require the use of
* "hand-over-hand" or "chain locking": you
* acquire the lock of node A, then node B, then release A and acquire
* C, then release B and acquire D and so on. Implementations of the
* {@code Lock} interface enable the use of such techniques by
* allowing a lock to be acquired and released in different scopes,
* and allowing multiple locks to be acquired and released in any
* order.
*
* <p>With this increased flexibility comes additional
* responsibility. The absence of block-structured locking removes the
* automatic release of locks that occurs with {@code synchronized}
* methods and statements. In most cases, the following idiom
* should be used:
*
* <pre> {@code
* Lock l = ...;
* l.lock();
* try {
* // access the resource protected by this lock
* } finally {
* l.unlock();
* }}</pre>
*
* When locking and unlocking occur in different scopes, care must be
* taken to ensure that all code that is executed while the lock is
* held is protected by try-finally or try-catch to ensure that the
* lock is released when necessary.
*
* <p>{@code Lock} implementations provide additional functionality
* over the use of {@code synchronized} methods and statements by
* providing a non-blocking attempt to acquire a lock ({@link
* #tryLock()}), an attempt to acquire the lock that can be
* interrupted ({@link #lockInterruptibly}, and an attempt to acquire
* the lock that can timeout ({@link #tryLock(long, TimeUnit)}).
*
* <p>A {@code Lock} class can also provide behavior and semantics
* that is quite different from that of the implicit monitor lock,
* such as guaranteed ordering, non-reentrant usage, or deadlock
* detection. If an implementation provides such specialized semantics
* then the implementation must document those semantics.
*
* <p>Note that {@code Lock} instances are just normal objects and can
* themselves be used as the target in a {@code synchronized} statement.
* Acquiring the
* monitor lock of a {@code Lock} instance has no specified relationship
* with invoking any of the {@link #lock} methods of that instance.
* It is recommended that to avoid confusion you never use {@code Lock}
* instances in this way, except within their own implementation.
*
* <p>Except where noted, passing a {@code null} value for any
* parameter will result in a {@link NullPointerException} being
* thrown.
*
* <h3>Memory Synchronization</h3>
*
* <p>All {@code Lock} implementations <em>must</em> enforce the same
* memory synchronization semantics as provided by the built-in monitor
* lock, as described in
* <a href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4">
* The Java Language Specification (17.4 Memory Model)</a>:
* <ul>
* <li>A successful {@code lock} operation has the same memory
* synchronization effects as a successful <em>Lock</em> action.
* <li>A successful {@code unlock} operation has the same
* memory synchronization effects as a successful <em>Unlock</em> action.
* </ul>
*
* Unsuccessful locking and unlocking operations, and reentrant
* locking/unlocking operations, do not require any memory
* synchronization effects.
*
* <h3>Implementation Considerations</h3>
*
* <p>The three forms of lock acquisition (interruptible,
* non-interruptible, and timed) may differ in their performance
* characteristics, ordering guarantees, or other implementation
* qualities. Further, the ability to interrupt the <em>ongoing</em>
* acquisition of a lock may not be available in a given {@code Lock}
* class. Consequently, an implementation is not required to define
* exactly the same guarantees or semantics for all three forms of
* lock acquisition, nor is it required to support interruption of an
* ongoing lock acquisition. An implementation is required to clearly
* document the semantics and guarantees provided by each of the
* locking methods. It must also obey the interruption semantics as
* defined in this interface, to the extent that interruption of lock
* acquisition is supported: which is either totally, or only on
* method entry.
*
* <p>As interruption generally implies cancellation, and checks for
* interruption are often infrequent, an implementation can favor responding
* to an interrupt over normal method return. This is true even if it can be
* shown that the interrupt occurred after another action may have unblocked
* the thread. An implementation should document this behavior.
*
* @see ReentrantLock
* @see Condition
* @see ReadWriteLock
*
* @since 1.5
* @author Doug Lea
*/
public interface Lock {
/**
* Acquires the lock.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until the
* lock has been acquired.
*
* <p><b>Implementation Considerations</b>
*
* <p>A {@code Lock} implementation may be able to detect erroneous use
* of the lock, such as an invocation that would cause deadlock, and
* may throw an (unchecked) exception in such circumstances. The
* circumstances and the exception type must be documented by that
* {@code Lock} implementation.
*/
void lock();
/**
* Acquires the lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the lock if it is available and returns immediately.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
*
* <ul>
* <li>The lock is acquired by the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread, and interruption of lock acquisition is supported.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while acquiring the
* lock, and interruption of lock acquisition is supported,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p><b>Implementation Considerations</b>
*
* <p>The ability to interrupt a lock acquisition in some
* implementations may not be possible, and if possible may be an
* expensive operation. The programmer should be aware that this
* may be the case. An implementation should document when this is
* the case.
*
* <p>An implementation can favor responding to an interrupt over
* normal method return.
*
* <p>A {@code Lock} implementation may be able to detect
* erroneous use of the lock, such as an invocation that would
* cause deadlock, and may throw an (unchecked) exception in such
* circumstances. The circumstances and the exception type must
* be documented by that {@code Lock} implementation.
*
* @throws InterruptedException if the current thread is
* interrupted while acquiring the lock (and interruption
* of lock acquisition is supported)
*/
void lockInterruptibly() throws InterruptedException;
/**
* Acquires the lock only if it is free at the time of invocation.
*
* <p>Acquires the lock if it is available and returns immediately
* with the value {@code true}.
* If the lock is not available then this method will return
* immediately with the value {@code false}.
*
* <p>A typical usage idiom for this method would be:
* <pre> {@code
* Lock lock = ...;
* if (lock.tryLock()) {
* try {
* // manipulate protected state
* } finally {
* lock.unlock();
* }
* } else {
* // perform alternative actions
* }}</pre>
*
* This usage ensures that the lock is unlocked if it was acquired, and
* doesn't try to unlock if the lock was not acquired.
*
* @return {@code true} if the lock was acquired and
* {@code false} otherwise
*/
boolean tryLock();
/**
* Acquires the lock if it is free within the given waiting time and the
* current thread has not been {@linkplain Thread#interrupt interrupted}.
*
* <p>If the lock is available this method returns immediately
* with the value {@code true}.
* If the lock is not available then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
* <ul>
* <li>The lock is acquired by the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread, and interruption of lock acquisition is supported; or
* <li>The specified waiting time elapses
* </ul>
*
* <p>If the lock is acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while acquiring
* the lock, and interruption of lock acquisition is supported,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned.
* If the time is
* less than or equal to zero, the method will not wait at all.
*
* <p><b>Implementation Considerations</b>
*
* <p>The ability to interrupt a lock acquisition in some implementations
* may not be possible, and if possible may
* be an expensive operation.
* The programmer should be aware that this may be the case. An
* implementation should document when this is the case.
*
* <p>An implementation can favor responding to an interrupt over normal
* method return, or reporting a timeout.
*
* <p>A {@code Lock} implementation may be able to detect
* erroneous use of the lock, such as an invocation that would cause
* deadlock, and may throw an (unchecked) exception in such circumstances.
* The circumstances and the exception type must be documented by that
* {@code Lock} implementation.
*
* @param time the maximum time to wait for the lock
* @param unit the time unit of the {@code time} argument
* @return {@code true} if the lock was acquired and {@code false}
* if the waiting time elapsed before the lock was acquired
*
* @throws InterruptedException if the current thread is interrupted
* while acquiring the lock (and interruption of lock
* acquisition is supported)
*/
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
/**
* Releases the lock.
*
* <p><b>Implementation Considerations</b>
*
* <p>A {@code Lock} implementation will usually impose
* restrictions on which thread can release a lock (typically only the
* holder of the lock can release it) and may throw
* an (unchecked) exception if the restriction is violated.
* Any restrictions and the exception
* type must be documented by that {@code Lock} implementation.
*/
void unlock();
/**
* Returns a new {@link Condition} instance that is bound to this
* {@code Lock} instance.
*
* <p>Before waiting on the condition the lock must be held by the
* current thread.
* A call to {@link Condition#await()} will atomically release the lock
* before waiting and re-acquire the lock before the wait returns.
*
* <p><b>Implementation Considerations</b>
*
* <p>The exact operation of the {@link Condition} instance depends on
* the {@code Lock} implementation and must be documented by that
* implementation.
*
* @return A new {@link Condition} instance for this {@code Lock} instance
* @throws UnsupportedOperationException if this {@code Lock}
* implementation does not support conditions
*/
Condition newCondition();
}
源码3：RLockAsync
package org.redisson.api;
import java.util.concurrent.TimeUnit;
/**
* Async interface for Lock object
*
* @author Nikita Koksharov
*
*/
public interface RLockAsync {
/**
* Unlocks the lock independently of state
*
* @return <code>true</code> if lock existed and now unlocked otherwise <code>false</code>
*/
RFuture<Boolean> forceUnlockAsync();
/**
* Unlocks the lock
*
* @return void
*/
RFuture<Void> unlockAsync();
/**
* Unlocks the lock. Throws {@link IllegalMonitorStateException}
* if lock isn't locked by thread with specified <code>threadId</code>.
*
* @param threadId id of thread
* @return void
*/
RFuture<Void> unlockAsync(long threadId);
/**
* Tries to acquire the lock.
*
* @return <code>true</code> if lock acquired otherwise <code>false</code>
*/
RFuture<Boolean> tryLockAsync();
/**
* Acquires the lock.
*
* @return void
*/
RFuture<Void> lockAsync();
/**
* Acquires the lock by thread with specified <code>threadId</code>.
*
* @param threadId id of thread
* @return void
*/
RFuture<Void> lockAsync(long threadId);
/**
* Acquires the lock.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until the
* lock has been acquired.
*
* If the lock is acquired, it is held until <code>unlock</code> is invoked,
* or until leaseTime milliseconds have passed
* since the lock was granted - whichever comes first.
*
* @param leaseTime the maximum time to hold the lock after granting it,
* before automatically releasing it if it hasn't already been released by invoking <code>unlock</code>.
* If leaseTime is -1, hold the lock until explicitly unlocked.
* @param unit the time unit of the {@code leaseTime} argument
* @return void
*/
RFuture<Void> lockAsync(long leaseTime, TimeUnit unit);
/**
* Acquires the lock by thread with specified <code>threadId</code>.
*
* <p>If the lock is not available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until the
* lock has been acquired.
*
* If the lock is acquired, it is held until <code>unlock</code> is invoked,
* or until leaseTime milliseconds have passed
* since the lock was granted - whichever comes first.
*
* @param leaseTime the maximum time to hold the lock after granting it,
* before automatically releasing it if it hasn't already been released by invoking <code>unlock</code>.
* If leaseTime is -1, hold the lock until explicitly unlocked.
* @param unit the time unit of the {@code leaseTime} argument
* @param threadId id of thread
* @return void
*/
RFuture<Void> lockAsync(long leaseTime, TimeUnit unit, long threadId);
/**
* Tries to acquire the lock by thread with specified <code>threadId</code>.
*
* @param threadId id of thread
* @return <code>true</code> if lock acquired otherwise <code>false</code>
*/
RFuture<Boolean> tryLockAsync(long threadId);
/**
* Tries to acquire the lock. If the lock is not available waits up
* to specified <code>waitTime</code> time interval to acquire it.
*
* @param waitTime interval to acquire lock
* @param unit the time unit of the {@code waitTime} argument
* @return <code>true</code> if lock acquired otherwise <code>false</code>
*/
RFuture<Boolean> tryLockAsync(long waitTime, TimeUnit unit);
/**
* Tries to acquire the lock. If the lock is not available waits
* up to specified <code>waitTime</code> time interval to acquire it.
* Lock will be release automatically after defined <code>leaseTime</code> interval.
*
* @param waitTime time interval to acquire lock
* @param leaseTime time interval after which lock will be released automatically
* @param unit the time unit of the {@code waitTime} and {@code leaseTime} arguments
* @return <code>true</code> if lock acquired otherwise <code>false</code>
*/
RFuture<Boolean> tryLockAsync(long waitTime, long leaseTime, TimeUnit unit);
/**
* Tries to acquire the lock by thread with specified <code>threadId</code>. If the lock is not available waits
* up to specified <code>waitTime</code> time interval to acquire it.
* Lock will be release automatically after defined <code>leaseTime</code> interval.
*
* @param threadId id of thread
* @param waitTime time interval to acquire lock
* @param leaseTime time interval after which lock will be released automatically
* @param unit the time unit of the {@code waitTime} and {@code leaseTime} arguments
* @return <code>true</code> if lock acquired otherwise <code>false</code>
*/
RFuture<Boolean> tryLockAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId);
/**
* Number of holds on this lock by the current thread
*
* @return holds or <code>0</code> if this lock is not held by current thread
*/
RFuture<Integer> getHoldCountAsync();
/**
* Checks if this lock locked by any thread
*
* @return <code>true</code> if locked otherwise <code>false</code>
*/
RFuture<Boolean> isLockedAsync();
/**
* Remaining time to live of this lock
*
* @return time in milliseconds
* -2 if the lock does not exist.
* -1 if the lock exists but has no associated expire.
*/
RFuture<Long> remainTimeToLiveAsync();
}
三、不同类型的分布式锁
-
可重入锁（Reentrant Lock）
-
公平锁（Fair Lock）
-
联锁（MultiLock）
-
红锁（RedLock）
-
读写锁（ReadWriteLock）
-
信号量（Semaphore）
-
可过期性信号量（PermitExpirableSemaphore）
-
闭锁（CountDownLatch）