摘要：如果為負值，表示不運行檢測線程。默認為策略的類名，默認為這里就用到了上面提到的兩個參數對象池原理分析避免泄漏配置參數詳解，以及資源回收，從池中獲取資源，將資源返還給池邏輯解析

本文主要解析一下apache common pools下的GenericObjectPool的參數設置

commons-pool2-2.4.2-sources.jar!/org/apache/commons/pool2/impl/GenericObjectPool.java

public class GenericObjectPool extends BaseGenericObjectPool
        implements ObjectPool, GenericObjectPoolMXBean, UsageTracking {
   //......
}        

默認配置見
commons-pool2-2.4.2-sources.jar!/org/apache/commons/pool2/impl/GenericObjectPoolConfig.java

public class GenericObjectPoolConfig extends BaseObjectPoolConfig {

    /**
     * The default value for the {@code maxTotal} configuration attribute.
     * @see GenericObjectPool#getMaxTotal()
     */
    public static final int DEFAULT_MAX_TOTAL = 8;

    /**
     * The default value for the {@code maxIdle} configuration attribute.
     * @see GenericObjectPool#getMaxIdle()
     */
    public static final int DEFAULT_MAX_IDLE = 8;

    /**
     * The default value for the {@code minIdle} configuration attribute.
     * @see GenericObjectPool#getMinIdle()
     */
    public static final int DEFAULT_MIN_IDLE = 0;


    private int maxTotal = DEFAULT_MAX_TOTAL;

    private int maxIdle = DEFAULT_MAX_IDLE;

    private int minIdle = DEFAULT_MIN_IDLE;

    //......
}

lifo
GenericObjectPool 提供了后進先出(LIFO)與先進先出(FIFO)兩種行為模式的池。默認為true，即當池中有空閑可用的對象時，調用borrowObject方法會返回最近（后進）的實例

fairness
當從池中獲取資源或者將資源還回池中時 是否使用java.util.concurrent.locks.ReentrantLock.ReentrantLock 的公平鎖機制,默認為false

maxTotal
鏈接池中最大連接數,默認為8

maxIdle
鏈接池中最大空閑的連接數,默認也為8

minIdle
連接池中最少空閑的連接數,默認為0

maxWaitMillis
當連接池資源耗盡時，等待時間，超出則拋異常，默認為-1即永不超時

blockWhenExhausted
當這個值為true的時候，maxWaitMillis參數才能生效。為false的時候，當連接池沒資源，則立馬拋異常。默認為true

testOnCreate
默認false，create的時候檢測是有有效，如果無效則從連接池中移除，并嘗試獲取繼續獲取

testOnBorrow
默認false，borrow的時候檢測是有有效，如果無效則從連接池中移除，并嘗試獲取繼續獲取

testOnReturn
默認false，return的時候檢測是有有效，如果無效則從連接池中移除，并嘗試獲取繼續獲取

testWhileIdle
默認false，在evictor線程里頭，當evictionPolicy.evict方法返回false時，而且testWhileIdle為true的時候則檢測是否有效，如果無效則移除

timeBetweenEvictionRunsMillis
空閑鏈接檢測線程檢測的周期，毫秒數。如果為負值，表示不運行檢測線程。默認為-1.

commons-pool2-2.4.2-sources.jar!/org/apache/commons/pool2/impl/GenericObjectPool.java

public GenericObjectPool(PooledObjectFactory factory,
            GenericObjectPoolConfig config) {

        super(config, ONAME_BASE, config.getJmxNamePrefix());

        if (factory == null) {
            jmxUnregister(); // tidy up
            throw new IllegalArgumentException("factory may not be null");
        }
        this.factory = factory;

        idleObjects = new LinkedBlockingDeque>(config.getFairness());

        setConfig(config);

        startEvictor(getTimeBetweenEvictionRunsMillis());
    }

commons-pool2-2.4.2-sources.jar!/org/apache/commons/pool2/impl/BaseGenericObjectPool.java

/**
     * The idle object evictor {@link TimerTask}.
     *
     * @see GenericKeyedObjectPool#setTimeBetweenEvictionRunsMillis
     */
    class Evictor extends TimerTask {
        /**
         * Run pool maintenance.  Evict objects qualifying for eviction and then
         * ensure that the minimum number of idle instances are available.
         * Since the Timer that invokes Evictors is shared for all Pools but
         * pools may exist in different class loaders, the Evictor ensures that
         * any actions taken are under the class loader of the factory
         * associated with the pool.
         */
        @Override
        public void run() {
            ClassLoader savedClassLoader =
                    Thread.currentThread().getContextClassLoader();
            try {
                if (factoryClassLoader != null) {
                    // Set the class loader for the factory
                    ClassLoader cl = factoryClassLoader.get();
                    if (cl == null) {
                        // The pool has been dereferenced and the class loader
                        // GC"d. Cancel this timer so the pool can be GC"d as
                        // well.
                        cancel();
                        return;
                    }
                    Thread.currentThread().setContextClassLoader(cl);
                }

                // Evict from the pool
                try {
                    evict();
                } catch(Exception e) {
                    swallowException(e);
                } catch(OutOfMemoryError oome) {
                    // Log problem but give evictor thread a chance to continue
                    // in case error is recoverable
                    oome.printStackTrace(System.err);
                }
                // Re-create idle instances.
                try {
                    ensureMinIdle();
                } catch (Exception e) {
                    swallowException(e);
                }
            } finally {
                // Restore the previous CCL
                Thread.currentThread().setContextClassLoader(savedClassLoader);
            }
        }
    }

numTestsPerEvictionRun
在每次空閑連接回收器線程(如果有)運行時檢查的連接數量，默認為3

private int getNumTests() {
        int numTestsPerEvictionRun = getNumTestsPerEvictionRun();
        if (numTestsPerEvictionRun >= 0) {
            return Math.min(numTestsPerEvictionRun, idleObjects.size());
        } else {
            return (int) (Math.ceil(idleObjects.size() /
                    Math.abs((double) numTestsPerEvictionRun)));
        }
    }

minEvictableIdleTimeMillis
連接空閑的最小時間，達到此值后空閑連接將可能會被移除。默認為1000L 60L 30L

softMinEvictableIdleTimeMillis
連接空閑的最小時間，達到此值后空閑鏈接將會被移除，且保留minIdle個空閑連接數。默認為-1.

evictionPolicyClassName
evict策略的類名，默認為org.apache.commons.pool2.impl.DefaultEvictionPolicy

public class DefaultEvictionPolicy implements EvictionPolicy {

    @Override
    public boolean evict(EvictionConfig config, PooledObject underTest,
            int idleCount) {

        if ((config.getIdleSoftEvictTime() < underTest.getIdleTimeMillis() &&
                config.getMinIdle() < idleCount) ||
                config.getIdleEvictTime() < underTest.getIdleTimeMillis()) {
            return true;
        }
        return false;
    }
}

這里就用到了上面提到的兩個參數

Apache commons-pool對象池原理分析

GenericObjectPool 避免泄漏

apache-common-pool2（配置參數詳解，以及資源回收，從池中獲取資源，將資源返還給池 邏輯解析）