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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
    
      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:     *     * 
 {@code     * if (map.get(key) == null) {     *     V newValue = mappingFunction.apply(key);     *     if (newValue != null)     *         return map.putIfAbsent(key, newValue);     * }     * }
     *     * The default implementation may retry these steps when multiple     * threads attempt updates including potentially calling the mapping     * function multiple times.     *     * 
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 must     * override this default implementation.     *     * @throws UnsupportedOperationException {@inheritDoc}     * @throws ClassCastException {@inheritDoc}     * @throws NullPointerException {@inheritDoc}     * @since 1.8     */    @Override    default V computeIfAbsent(K key,            Function
      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
    
      putIfAbsentAsync(final K key, final V value) {        checkKey(key);        checkValue(key);                RFuture
     
       future = putIfAbsentOperationAsync(key, value);        if (hasNoWriter()) {            return future;        }                MapWriterTask
      
        listener = new MapWriterTask
       
        () {            @Override            public void execute() {                options.getWriter().write(key, value);            }                        @Override            protected boolean condition(Future
        
          future) {                return future.getNow() == null;            }        };        return mapWriterFuture(future, listener);    }    protected boolean hasNoWriter() {        return options == null || options.getWriter() == null;    }    protected RFuture
         
           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.
          
         
        
       
      
     
    

• 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。

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