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本文次要钻研一下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<T> 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(); }- borrowObject办法会开启一个while循环，条件是p为null，也就是要获取到p或者是外部本人跳出循环；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进去的为null，则阻塞期待takeFirst或者pollFirst(borrowMaxWaitDuration)，如果还是null则抛出NoSuchElementException；如果blockWhenExhausted为false然而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)不胜利则返回null；接着执行factory.activateObject(p)办法，如果出现异常则destory掉(jedis这里只是在db不一样的时候会从新select，默认能够了解为空操作)，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则间接抛出NoSuchElementException异样，跳出循环；只有在不为null且allocate失败的时候会重置为null持续循环；另外如果是create进去的然而activate不胜利也会抛出NoSuchElementException异样，跳出循环
create
/** * Attempts to create a new wrapped pooled object. * <p> * If there are {@link #getMaxTotal()} objects already in circulation * or in process of being created, this method returns null. * </p> * * @return The new wrapped pooled object * * @throws Exception if the object factory's {@code makeObject} fails */ private PooledObject<T> 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<T> 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; }create办法不会判断createCount，如果超出则返回null，如果期待超出maxWait也会返回null；如果判断要创立则通过factory.makeObject()，另外针对testOnCreate且validateObject不通过的也返回null，如果是有异样则间接抛出
makeObject
redis/clients/jedis/JedisFactory.java
@Override public PooledObject<Jedis> 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; } }JedisFactory的makeObject会创立Jedis而后执行connect，如果有JedisException则抛出，这个也会间接跳出borrowObject的循环，间接给到调用方
activateObject
redis/clients/jedis/JedisFactory.java
public void activateObject(PooledObject<Jedis> pooledJedis) throws Exception { final BinaryJedis jedis = pooledJedis.getObject(); if (jedis.getDB() != clientConfig.getDatabase()) { jedis.select(clientConfig.getDatabase()); } }JedisFactory的activateObject就是判断db跟配置的是不是一样，不一样则从新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 }testOnBorrow的逻辑就是执行validateObject办法，如果是ThreadDeath或者VirtualMachineError才会从新抛出，否则吞掉，之后判断validate后果，如果不胜利则执行destory办法，从新设置为null，然而如果这个是create进去的则抛出NoSuchElementException
小结
jedis的borrow行为是在while循环外头去获取的，个别是在allocate变更状态不胜利(PooledObjectState.IDLE-->PooledObjectState.ALLOCATED)的时候会从新设置null，持续循环
- 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，这个也会间接跳出borrowObject的循环，间接给到调用方 - borrow进去不会null的执行activateObject，jedis这里只是在db不一样的时候会从新select，默认能够了解为空操作
最初是testOnBorrow的逻辑，如果有异样，则针对create进去的则抛出NoSuchElementException跳出循环，否则重置为null持续循环
总结一下就是如果是create有异样(
JedisException)则间接抛出，如果borrow不到(即便通过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进去的，也会抛出NoSuchElementException(Unable to validate object)