聊聊jedis的borrow行为

序

本文主要研究一下jedis的borrow行为

borrowObject

org/apache/commons/pool2/impl/GenericObjectPool.java

    public T borrowObject(final Duration borrowMaxWaitDuration) throws Exception {
        assertOpen();

        final AbandonedConfig ac = this.abandonedConfig;
        if (ac != null && ac.getRemoveAbandonedOnBorrow() && (getNumIdle() < 2) &&
                (getNumActive() > getMaxTotal() - 3)) {
            removeAbandoned(ac);
        }

        PooledObject p = null;

        // Get local copy of current config so it is consistent for entire
        // method execution
        final boolean blockWhenExhausted = getBlockWhenExhausted();

        boolean create;
        final long waitTimeMillis = System.currentTimeMillis();

        while (p == null) {
            create = false;
            p = idleObjects.pollFirst();
            if (p == null) {
                p = create();
                if (p != null) {
                    create = true;
                }
            }
            if (blockWhenExhausted) {
                if (p == null) {
                    if (borrowMaxWaitDuration.isNegative()) {
                        p = idleObjects.takeFirst();
                    } else {
                        p = idleObjects.pollFirst(borrowMaxWaitDuration);
                    }
                }
                if (p == null) {
                    throw new NoSuchElementException(appendStats(
                            "Timeout waiting for idle object, borrowMaxWaitDuration=" + borrowMaxWaitDuration));
                }
            } else if (p == null) {
                throw new NoSuchElementException(appendStats("Pool exhausted"));
            }
            if (!p.allocate()) {
                p = null;
            }

            if (p != null) {
                try {
                    factory.activateObject(p);
                } catch (final Exception e) {
                    try {
                        destroy(p, DestroyMode.NORMAL);
                    } catch (final Exception e1) {
                        // Ignore - activation failure is more important
                    }
                    p = null;
                    if (create) {
                        final NoSuchElementException nsee = new NoSuchElementException(
                                appendStats("Unable to activate object"));
                        nsee.initCause(e);
                        throw nsee;
                    }
                }
                if (p != null && getTestOnBorrow()) {
                    boolean validate = false;
                    Throwable validationThrowable = null;
                    try {
                        validate = factory.validateObject(p);
                    } catch (final Throwable t) {
                        PoolUtils.checkRethrow(t);
                        validationThrowable = t;
                    }
                    if (!validate) {
                        try {
                            destroy(p, DestroyMode.NORMAL);
                            destroyedByBorrowValidationCount.incrementAndGet();
                        } catch (final Exception e) {
                            // Ignore - validation failure is more important
                        }
                        p = null;
                        if (create) {
                            final NoSuchElementException nsee = new NoSuchElementException(
                                    appendStats("Unable to validate object"));
                            nsee.initCause(validationThrowable);
                            throw nsee;
                        }
                    }
                }
            }
        }

        updateStatsBorrow(p, Duration.ofMillis(System.currentTimeMillis() - waitTimeMillis));

        return p.getObject();
    }
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• borrowObject方法会开启一个while循环，条件是p为null，也就是要获取到p或者是内部自己跳出循环；idleObjects.pollFirst()从连接池获取，如果为null则执行create，之后是blockWhenExhausted的判断逻辑，如果create出来的为null，则阻塞等待takeFirst或者pollFirst(borrowMaxWaitDuration)，如果还是null则抛出NoSuchElementException；如果blockWhenExhausted为false但是create为null则抛出Pool exhausted
• 如果不是null，则再次确认下object的状态，如果变更状态(PooledObjectState.IDLE-->PooledObjectState.ALLOCATED)不成功则返回null；接着执行factory.activateObject§方法，如果出现异常则destory掉(jedis这里只是在db不一样的时候会重新select，默认可以理解为空操作)，紧接着是testOnBorrow的逻辑
• 这里就是如果idleObjects.pollFirst()为null会触发create，如果还是null则直接抛出NoSuchElementException异常，跳出循环；只有在不为null且allocate失败的时候会重置为null继续循环；另外如果是create出来的但是activate不成功也会抛出NoSuchElementException异常，跳出循环

create

    /**
     * Attempts to create a new wrapped pooled object.
     * 

     * If there are {@link #getMaxTotal()} objects already in circulation
     * or in process of being created, this method returns null.
     * 
     *
     * @return The new wrapped pooled object
     *
     * @throws Exception if the object factory's {@code makeObject} fails
     */
    private PooledObject create() throws Exception {
        int localMaxTotal = getMaxTotal();
        // This simplifies the code later in this method
        if (localMaxTotal < 0) {
            localMaxTotal = Integer.MAX_VALUE;
        }

        final long localStartTimeMillis = System.currentTimeMillis();
        final long localMaxWaitTimeMillis = Math.max(getMaxWaitDuration().toMillis(), 0);

        // Flag that indicates if create should:
        // - TRUE:  call the factory to create an object
        // - FALSE: return null
        // - null:  loop and re-test the condition that determines whether to
        //          call the factory
        Boolean create = null;
        while (create == null) {
            synchronized (makeObjectCountLock) {
                final long newCreateCount = createCount.incrementAndGet();
                if (newCreateCount > localMaxTotal) {
                    // The pool is currently at capacity or in the process of
                    // making enough new objects to take it to capacity.
                    createCount.decrementAndGet();
                    if (makeObjectCount == 0) {
                        // There are no makeObject() calls in progress so the
                        // pool is at capacity. Do not attempt to create a new
                        // object. Return and wait for an object to be returned
                        create = Boolean.FALSE;
                    } else {
                        // There are makeObject() calls in progress that might
                        // bring the pool to capacity. Those calls might also
                        // fail so wait until they complete and then re-test if
                        // the pool is at capacity or not.
                        makeObjectCountLock.wait(localMaxWaitTimeMillis);
                    }
                } else {
                    // The pool is not at capacity. Create a new object.
                    makeObjectCount++;
                    create = Boolean.TRUE;
                }
            }

            // Do not block more if maxWaitTimeMillis is set.
            if (create == null &&
                (localMaxWaitTimeMillis > 0 &&
                 System.currentTimeMillis() - localStartTimeMillis >= localMaxWaitTimeMillis)) {
                create = Boolean.FALSE;
            }
        }

        if (!create.booleanValue()) {
            return null;
        }

        final PooledObject p;
        try {
            p = factory.makeObject();
            if (getTestOnCreate() && !factory.validateObject(p)) {
                createCount.decrementAndGet();
                return null;
            }
        } catch (final Throwable e) {
            createCount.decrementAndGet();
            throw e;
        } finally {
            synchronized (makeObjectCountLock) {
                makeObjectCount--;
                makeObjectCountLock.notifyAll();
            }
        }

        final AbandonedConfig ac = this.abandonedConfig;
        if (ac != null && ac.getLogAbandoned()) {
            p.setLogAbandoned(true);
            p.setRequireFullStackTrace(ac.getRequireFullStackTrace());
        }

        createdCount.incrementAndGet();
        allObjects.put(new IdentityWrapper<>(p.getObject()), p);
        return p;
    }
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create方法不会判断createCount，如果超出则返回null，如果等待超出maxWait也会返回null；如果判断要创建则通过factory.makeObject()，另外针对testOnCreate且validateObject不通过的也返回null，如果是有异常则直接抛出

makeObject

redis/clients/jedis/JedisFactory.java

  @Override
  public PooledObject makeObject() throws Exception {
    Jedis jedis = null;
    try {
      jedis = new Jedis(jedisSocketFactory, clientConfig);
      jedis.connect();
      return new DefaultPooledObject<>(jedis);
    } catch (JedisException je) {
      if (jedis != null) {
        try {
          jedis.quit();
        } catch (RuntimeException e) {
          logger.warn("Error while QUIT", e);
        }
        try {
          jedis.close();
        } catch (RuntimeException e) {
          logger.warn("Error while close", e);
        }
      }
      throw je;
    }
  }
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JedisFactory的makeObject会创建Jedis然后执行connect，如果有JedisException则抛出，这个也会直接跳出borrowObject的循环，直接给到调用方

activateObject

redis/clients/jedis/JedisFactory.java

  public void activateObject(PooledObject pooledJedis) throws Exception {
    final BinaryJedis jedis = pooledJedis.getObject();
    if (jedis.getDB() != clientConfig.getDatabase()) {
      jedis.select(clientConfig.getDatabase());
    }
  }
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JedisFactory的activateObject就是判断db跟配置的是不是一样，不一样则重新select

testOnBorrow

                if (p != null && getTestOnBorrow()) {
                    boolean validate = false;
                    Throwable validationThrowable = null;
                    try {
                        validate = factory.validateObject(p);
                    } catch (final Throwable t) {
                        PoolUtils.checkRethrow(t);
                        validationThrowable = t;
                    }
                    if (!validate) {
                        try {
                            destroy(p, DestroyMode.NORMAL);
                            destroyedByBorrowValidationCount.incrementAndGet();
                        } catch (final Exception e) {
                            // Ignore - validation failure is more important
                        }
                        p = null;
                        if (create) {
                            final NoSuchElementException nsee = new NoSuchElementException(
                                    appendStats("Unable to validate object"));
                            nsee.initCause(validationThrowable);
                            throw nsee;
                        }
                    }
                }

    public static void checkRethrow(final Throwable t) {
        if (t instanceof ThreadDeath) {
            throw (ThreadDeath) t;
        }
        if (t instanceof VirtualMachineError) {
            throw (VirtualMachineError) t;
        }
        // All other instances of Throwable will be silently swallowed
    }
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testOnBorrow的逻辑就是执行validateObject方法，如果是ThreadDeath或者VirtualMachineError才会重新抛出，否则吞掉，之后判断validate结果，如果不成功则执行destory方法，重新设置为null，但是如果这个是create出来的则抛出NoSuchElementException

小结

jedis的borrow行为是在while循环里头去获取的，一般是在allocate变更状态不成功(PooledObjectState.IDLE-->PooledObjectState.ALLOCATED)的时候会重新设置null，继续循环

• idleObjects.pollFirst()为null会触发create，如果还是null则抛出NoSuchElementException(Pool exhausted)跳出循环；如果blockWhenExhausted为true，block之后获取到的还是null，也会抛出NoSuchElementException(Timeout waiting for idle object)跳出循环；如果触发create操作，且create抛出JedisException，这个也会直接跳出borrowObject的循环，直接给到调用方
• borrow出来不会null的执行activateObject，jedis这里只是在db不一样的时候会重新select，默认可以理解为空操作
• 最后是testOnBorrow的逻辑，如果有异常，则针对create出来的则抛出NoSuchElementException跳出循环，否则重置为null继续循环

总结一下就是如果是create有异常(JedisException)则直接抛出，如果borrow不到(即使经过create)也会抛出NoSuchElementException(具体可能是Pool exhausted或者Timeout waiting for idle object)，如果有testOnBorrow不通过且是create出来的，也会抛出NoSuchElementException(Unable to validate object)