jedis连接池参数minEvictableIdleTimeMillis和softMinEvictableIdleTimeMillis探索
我们通常在使用JedisPoolConfig进行连接池配置的时候，minEvictableIdleTimeMillis和softMinEvictableIdleTimeMillis这两个参数经常会不懂其含义，
查各种资料也没有非常明确的说到底该如何设置，即使知道如何设置，也不知道其原理，只知道这两个参数是和逐出线程有关的。下面根据源码进行探索。
我们通常是通过JedisPool构造线程池，追溯其父类的创建过程，发现Pool<T>这个泛型类的构造方法调用过程如下：
public Pool(GenericObjectPoolConfig poolConfig, PooledObjectFactory<T> factory) {
this.initPool(poolConfig, factory);
}
public void initPool(GenericObjectPoolConfig poolConfig, PooledObjectFactory<T> factory) {
if(this.internalPool != null) {
try {
this.closeInternalPool();
} catch (Exception var4) {
;
}
}
this.internalPool = new GenericObjectPool(factory, poolConfig);
}
发现其创建了一个GenericObjectPool对象，构造方法如下：
public GenericObjectPool(PooledObjectFactory<T> factory, GenericObjectPoolConfig config) {
super(config, "org.apache.commons.pool2:type=GenericObjectPool,name=", config.getJmxNamePrefix());
this.factoryType = null;
this.maxIdle = 8;
this.minIdle = 0;
this.allObjects = new ConcurrentHashMap();
this.createCount = new AtomicLong(0L);
this.abandonedConfig = null;
if(factory == null) {
this.jmxUnregister();
throw new IllegalArgumentException("factory may not be null");
} else {
this.factory = factory;
this.idleObjects = new LinkedBlockingDeque(config.getFairness());
this.setConfig(config);
this.startEvictor(this.getTimeBetweenEvictionRunsMillis());
}
}
其中this.startEvictor(this.getTimeBetweenEvictionRunsMillis());方法的调用，正是开启逐出线程运行的作用，
我们可以发现，源码通过周期性的调度逐出任务（timeBetweenEvictionRunsMillis大于0时），将空闲的连接逐出线程池。
final void startEvictor(long delay) {
Object var3 = this.evictionLock;
synchronized(this.evictionLock) {
if(null != this.evictor) {
EvictionTimer.cancel(this.evictor);
this.evictor = null;
this.evictionIterator = null;
}
if(delay > 0L) {
this.evictor = new BaseGenericObjectPool.Evictor();
EvictionTimer.schedule(this.evictor, delay, delay);
}
}
}
下面将是我们今天研究的重点，this.evictor。
逐出有逐出策略，如果不配置则采用默认的逐出策略DefaultEvictionPolicy，其中的evict方法返回true时才执行逐出的操作
public class DefaultEvictionPolicy<T> implements EvictionPolicy<T> {
public DefaultEvictionPolicy() {
}
public boolean evict(EvictionConfig config, PooledObject<T> underTest, int idleCount) {
return config.getIdleSoftEvictTime() < underTest.getIdleTimeMillis() && config.getMinIdle() < idleCount || config.getIdleEvictTime() < underTest.getIdleTimeMillis();
}
}
真正的逐出方法执行的是以下内容
public void evict() throws Exception {
this.assertOpen();
if(this.idleObjects.size() > 0) {
PooledObject<T> underTest = null;
EvictionPolicy<T> evictionPolicy = this.getEvictionPolicy();
Object var3 = this.evictionLock;
synchronized(this.evictionLock) {
EvictionConfig evictionConfig = new EvictionConfig(this.getMinEvictableIdleTimeMillis(), this.getSoftMinEvictableIdleTimeMillis(), this.getMinIdle());
boolean testWhileIdle = this.getTestWhileIdle();
int i = 0;
int m = this.getNumTests();
while(true) {
if(i >= m) {
break;
}
if(this.evictionIterator == null || !this.evictionIterator.hasNext()) {
this.evictionIterator = new EvictionIterator(this, this.idleObjects);
}
if(!this.evictionIterator.hasNext()) {
return;
}
label81: {
try {
underTest = this.evictionIterator.next();
} catch (NoSuchElementException var15) {
--i;
this.evictionIterator = null;
break label81;
}
if(!underTest.startEvictionTest()) {
--i;
} else {
boolean evict;
try {
evict = evictionPolicy.evict(evictionConfig, underTest, this.idleObjects.size());
} catch (Throwable var14) {
PoolUtils.checkRethrow(var14);
this.swallowException(new Exception(var14));
evict = false;
}
if(evict) {
this.destroy(underTest);
this.destroyedByEvictorCount.incrementAndGet();
} else {
if(testWhileIdle) {
boolean active = false;
try {
this.factory.activateObject(underTest);
active = true;
} catch (Exception var13) {
this.destroy(underTest);
this.destroyedByEvictorCount.incrementAndGet();
}
if(active) {
if(!this.factory.validateObject(underTest)) {
this.destroy(underTest);
this.destroyedByEvictorCount.incrementAndGet();
} else {
try {
this.factory.passivateObject(underTest);
} catch (Exception var12) {
this.destroy(underTest);
this.destroyedByEvictorCount.incrementAndGet();
}
}
}
}
if(!underTest.endEvictionTest(this.idleObjects)) {
;
}
}
}
}
++i;
}
}
}
AbandonedConfig ac = this.abandonedConfig;
if(ac != null && ac.getRemoveAbandonedOnMaintenance()) {
this.removeAbandoned(ac);
}
}
我们重点看两行代码，第8行是创建了逐出配置，根据你配置的minEvictableIdleTimeMillis和softMinEvictableIdleTimeMillis，如果存在负数，则设为long类型的最大值。
public EvictionConfig(long poolIdleEvictTime, long poolIdleSoftEvictTime, int minIdle) {
if(poolIdleEvictTime > 0L) {
this.idleEvictTime = poolIdleEvictTime;
} else {
this.idleEvictTime = 9223372036854775807L;
}
if(poolIdleSoftEvictTime > 0L) {
this.idleSoftEvictTime = poolIdleSoftEvictTime;
} else {
this.idleSoftEvictTime = 9223372036854775807L;
}
this.minIdle = minIdle;
}
再看第40行代码，再结合DefaultEvictionPolicy的evict方法，我们可以看到，真正的逐出依据是：
1.连接空闲时间大于softMinEvictableIdleTimeMillis并且当前连接池的空闲连接数大于最小空闲连接数minIdle；
2.连接空闲时间大于minEvictableIdleTimeMillis。
1或者2成立即可逐出，注意是或的关系。
所以，结论如下：
如果要连接池只根据softMinEvictableIdleTimeMillis进程逐出，那么需要将minEvictableIdleTimeMillis设置为负数（即最大值）；
如果要连接池只根据minEvictableIdleTimeMillis进程逐出，那么需要将softMinEvictableIdleTimeMillis设置为负数（即最大值），理论上设置minIdle很大也是可以的，但是实际上不行；
