Spymemcached源码 第三节 get请求分析

下面分析get请求逻辑

//根据key获取相应的值，并使用解码器进行解码
  public Object get(String key) {
    return get(key, transcoder);
  }
  
  //异步获取future，并经过get方法设置超时时间，获取结果
  public <T> T get(String key, Transcoder<T> tc) {
    try {
      return asyncGet(key, tc).get(operationTimeout, TimeUnit.MILLISECONDS);
    } catch (InterruptedException e) {
      throw new RuntimeException("Interrupted waiting for value", e);
    } catch (ExecutionException e) {
      if(e.getCause() instanceof CancellationException) {
        throw (CancellationException) e.getCause();
      } else {
        throw new RuntimeException("Exception waiting for value", e);
      }
    } catch (TimeoutException e) {
      throw new OperationTimeoutException("Timeout waiting for value: "
        + buildTimeoutMessage(operationTimeout, TimeUnit.MILLISECONDS), e);
    }
  }
  
   public <T> GetFuture<T> asyncGet(final String key, final Transcoder<T> tc) {

    final CountDownLatch latch = new CountDownLatch(1);
    final GetFuture<T> rv = new GetFuture<T>(latch, operationTimeout, key,
      executorService);
    //读取到END/n/r的时候前后调用receivedStatus，complete
    //获取到数据后调用gotData
    //执行顺序是gotData，receivedStatus，complete
    Operation op = opFact.get(key, new GetOperation.Callback() {
      private Future<T> val;

      @Override
      public void receivedStatus(OperationStatus status) {
        rv.set(val, status);
      }

      @Override
      public void gotData(String k, int flags, byte[] data) {
        assert key.equals(k) : "Wrong key returned";
        val =
            tcService.decode(tc, new CachedData(flags, data, tc.getMaxSize()));
      }

      @Override
      public void complete() {
        latch.countDown();
        rv.signalComplete();
      }
    });
    rv.setOperation(op);
    mconn.enqueueOperation(key, op);  ==》
    return rv;
  }
  
  
  protected void addOperation(final String key, final Operation o) {
    MemcachedNode placeIn = null;
    //根据key路由，默认负载算法是根据key hash取模
    MemcachedNode primary = locator.getPrimary(key);
    //节点可用或者失败处理模式为重试机制
    if (primary.isActive() || failureMode == FailureMode.Retry) {
      placeIn = primary;
    } else if (failureMode == FailureMode.Cancel) {
      //节点不可用且重试模式为当即中止
      o.cancel();
    } else {
      //若是primary不可用，且FailureMode为Redistribute则选择一个能够节点处理
      Iterator<MemcachedNode> i = locator.getSequence(key);
      while (placeIn == null && i.hasNext()) {
        MemcachedNode n = i.next();
        if (n.isActive()) {
          placeIn = n;
        }
      }

      if (placeIn == null) {
        placeIn = primary;
        this.getLogger().warn("Could not redistribute to another node, "
          + "retrying primary node for %s.", key);
      }
    }

    assert o.isCancelled() || placeIn != null : "No node found for key " + key;
    if (placeIn != null) {
    //将op添加到该节点的inputQ中，同时也把op放入到addedQueue中  
      addOperation(placeIn, o);
    } else {
      assert o.isCancelled() : "No node found for " + key + " (and not "
        + "immediately cancelled)";
    }
  }

调用完asyncGet后获取到GetFuture，而后再调用GetFuture的get方法：

能够看出首先根据rc获取到Furure对象，而后经过Future get方法获取最终结果

public T get(long duration, TimeUnit units) throws InterruptedException,
      TimeoutException, ExecutionException {
    Future<T> v = rv.get(duration, units);
    return v == null ? null : v.get();
  }

再看rv.get(duration, units)方法：

public T get(long duration, TimeUnit units) throws InterruptedException,
      TimeoutException, ExecutionException {
    //阻塞，等待GetOperation.Callback()调用complete方法
    if (!latch.await(duration, units)) {
      // whenever timeout occurs, continuous timeout counter will increase by 1.
      //记录某个节点操做超时次数+1
      MemcachedConnection.opTimedOut(op);
      if (op != null) { // op can be null on a flush
        //标识超时
        op.timeOut();
      }
      throw new CheckedOperationTimeoutException(
          "Timed out waiting for operation", op);
    } else {
      // continuous timeout counter will be reset
      //重置某个节点操做超时次数为0
      MemcachedConnection.opSucceeded(op);
    }
    if (op != null && op.hasErrored()) {
      throw new ExecutionException(op.getException());
    }
    if (isCancelled()) {
      throw new ExecutionException(new CancellationException("Cancelled"));
    }
    if (op != null && op.isTimedOut()) {
      throw new ExecutionException(new CheckedOperationTimeoutException(
          "Operation timed out.", op));
    }

    /* TODO: re-add assertion that op.getState() == OperationState.COMPLETE */
    //获取GetOperation中Callbac中gotData方法中
    //tcService.decode(tc, new CachedData(flags, data,tc.getMaxSize()))生成的Future
    return objRef.get();
  }