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本文主要研究一下 redisson 的 RMap 的 computeIfAbsent 操作
实例
@Test
public void testRMapComputeIfAbsent(){
Config config = new Config();
config.useSingleServer()
.setAddress(“redis://192.168.99.100:6379”);
RedissonClient redisson = Redisson.create(config);
RMap<String, CityInfo> map = redisson.getMap(“anyMap”);
CityInfo bj = CityInfo.builder().name(“bj”).build();
CityInfo currentObject = map.computeIfAbsent(“bj”, k -> bj);
Assert.assertEquals(bj.toString(),currentObject.toString());
}
源码分析
ConcurrentMap.computeIfAbsent
java/util/concurrent/ConcurrentMap.java
/**
* {@inheritDoc}
*
* @implSpec
* The default implementation is equivalent to the following steps for this
* {@code map}, then returning the current value or {@code null} if now
* absent:
*
* <pre> {@code
* if (map.get(key) == null) {
* V newValue = mappingFunction.apply(key);
* if (newValue != null)
* return map.putIfAbsent(key, newValue);
* }
* }</pre>
*
* The default implementation may retry these steps when multiple
* threads attempt updates including potentially calling the mapping
* function multiple times.
*
* <p>This implementation assumes that the ConcurrentMap cannot contain null
* values and {@code get()} returning null unambiguously means the key is
* absent. Implementations which support null values <strong>must</strong>
* override this default implementation.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @since 1.8
*/
@Override
default V computeIfAbsent(K key,
Function<? super K, ? extends V> mappingFunction) {
Objects.requireNonNull(mappingFunction);
V v, newValue;
return ((v = get(key)) == null &&
(newValue = mappingFunction.apply(key)) != null &&
(v = putIfAbsent(key, newValue)) == null) ? newValue : v;
}
computeIfAbsent 当该 key 不存在时，返回的是新值，而非 null
computeIfAbsent 方法里头调用了 putIfAbsent
RedissonMap.putIfAbsent
redisson-3.8.1-sources.jar!/org/redisson/RedissonMap.java
@Override
public V putIfAbsent(K key, V value) {
return get(putIfAbsentAsync(key, value));
}
@Override
public RFuture<V> putIfAbsentAsync(final K key, final V value) {
checkKey(key);
checkValue(key);
RFuture<V> future = putIfAbsentOperationAsync(key, value);
if (hasNoWriter()) {
return future;
}
MapWriterTask<V> listener = new MapWriterTask<V>() {
@Override
public void execute() {
options.getWriter().write(key, value);
}
@Override
protected boolean condition(Future<V> future) {
return future.getNow() == null;
}
};
return mapWriterFuture(future, listener);
}
protected boolean hasNoWriter() {
return options == null || options.getWriter() == null;
}
protected RFuture<V> putIfAbsentOperationAsync(K key, V value) {
return commandExecutor.evalWriteAsync(getName(key), codec, RedisCommands.EVAL_MAP_VALUE,
“if redis.call(‘hsetnx’, KEYS[1], ARGV[1], ARGV[2]) == 1 then ”
+ “return nil ”
+ “else ”
+ “return redis.call(‘hget’, KEYS[1], ARGV[1]) ”
+ “end”,
Collections.<Object>singletonList(getName(key)), encodeMapKey(key), encodeMapValue(value));
}
protected <M> RFuture<M> mapWriterFuture(RFuture<M> future, final MapWriterTask<M> listener) {
if (options != null && options.getWriteMode() == WriteMode.WRITE_BEHIND) {
future.addListener(new MapWriteBehindListener<M>(commandExecutor, listener, writeBehindCurrentThreads, writeBehindTasks, options.getWriteBehindThreads()));
return future;
}
final RPromise<M> promise = new RedissonPromise<M>();
future.addListener(new FutureListener<M>() {
@Override
public void operationComplete(final Future<M> future) throws Exception {
if (!future.isSuccess()) {
promise.tryFailure(future.cause());
return;
}
if (listener.condition(future)) {
commandExecutor.getConnectionManager().getExecutor().execute(new Runnable() {
@Override
public void run() {
try {
listener.execute();
} catch (Exception e) {
promise.tryFailure(e);
return;
}
promise.trySuccess(future.getNow());
}
});
} else {
promise.trySuccess(future.getNow());
}
}
});
return promise;
}
RedissonMap 覆盖了 putIfAbsent 方法，这里调用的是 putIfAbsentAsync，该方法主要调用 putIfAbsentOperationAsync
putIfAbsentOperationAsync 使用了一段 lua 脚本来保证原子性，如果 hsetnx 之前的 key 不存在且设置成功则返回 nil，否则查找已有的值返回
putIfAbsentAsync 除了调用 putIfAbsentOperationAsync，还增加了 writer 的逻辑，如果有设置 writer，则会在 putIfAbsentOperationAsync 的 future 成功时回调触发 writer
writer 主要用于外部存储用，比如旁路存储一份到本地磁盘，有同步操作及异步操作两种模式
小结
redisson 对 redis 的数据结构操作进行了更进一步的封装，比如 redisson 的 RMap 实现了 java.util.concurrent.ConcurrentMap 接口和 java.util.Map 接口，实现了诸如 putIfAbsent 的方法，用 lua 脚本在服务端保证了操作的原子性。
doc
7. 分布式集合
阿里云专访 Redisson 作者 Rui Gu：构建开源企业级 Redis 客户端之路
