Why sleep mode connections are not reused by c3p0?

问题

I develop an web application using Spring MVC + MySQL. To manage JDBC connection I used c3p0 for connection pooling.

If I am using c3p0, What I suppose if, 5 connections are open in pool and all 5 connections are in sleep mode and if I request getConnection() from java then one of them should be returned as Connection. Am I correct?

If Yes Then consider following load testing scenario where I am getting Exception Connections could not be acquired from the underlying database!. Using Jmeter I started load test for 250 users in 15 seconds. I m continuously observing DB connections using SHOE FULL PROCESSLIST for first some minutes all is going well but as all the 250 users thrown to web server connections count is reached to 500 which is our maxPoolSize. So after this we are getting Connections could not be acquired from the underlying database!.

At this point if I execute SHOE FULL PROCESSLIST then I can see all 500 connections are in sleep mode If I am correct in above statement that any open connection that is in sleep mode will be returned by c3p0. Then why I am getting this exception.?

Here is my c3p0 properties

MINPOOLSIZE=10
ACQUIREINCREMENT=1
MAXPOOLSIZE=500
INITIALPOOLSIZE=10
NUMBERHELPERTHREAD=100
MAXIDLETIME=10
MAXSTATEMENT=20
MAXSTATEMENTPERCONNECTION=5
IDLECONNECTIONTESTPERIOD=120
ACQUIRERETRYATTEMPT=10
ACQUIRERETRYDELAY=100
AUTOCOMMITONCLOSE=false
BREAKAFTERACQUIREFAILURE=false
TESTCONNECTIONONCHECKOUT=true
TESTCONNECTIONONCHECKIN=true

Update

I found this warning before Connections could not be acquired from the underlying database! Exception

WARN : 30 Jun 2014 10:40:12.078 com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask.run(BasicResourcePool.java:1839) - com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask@e49561 -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (10). Last acquisition attempt exception: 

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.Util.getInstance(Util.java:386)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4187)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4119)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:927)
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1709)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1252)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2483)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2516)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2301)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:834)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
    at sun.reflect.GeneratedConstructorAccessor18.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:416)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:346)
    at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:183)
    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:172)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:188)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1074)
    at com.mchange.v2.resourcepool.BasicResourcePool.doAcquireAndDecrementPendingAcquiresWithinLockOnSuccess(BasicResourcePool.java:1061)
    at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
    at com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask.run(BasicResourcePool.java:1798)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:636)
WARN : 30 Jun 2014 10:40:12.078 com.mchange.v2.resourcepool.BasicResourcePool.forceKillAcquires(BasicResourcePool.java:882) - Having failed to acquire a resource, com.mchange.v2.resourcepool.BasicResourcePool@1035ff9 is interrupting all Threads waiting on a resource to check out. Will try again in response to new client requests.

来源：https://stackoverflow.com/questions/24451317/why-sleep-mode-connections-are-not-reused-by-c3p0