聊聊redisson的RMap的computeIfAbsent操作

序

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.