[2013-12-06 11:06:21,715] [C3P0PooledConnectionPoolManager[identityToken->2tl0n98y1iwg7cbdzzq7a|719f1f]-HelperThread-#2] DEBUG - com.mchange.v2.c3p0.impl.NewPooledConnection@484c6b closed by a client.
java.lang.Exception: DEBUG -- CLOSE BY CLIENT STACK TRACE
    at com.mchange.v2.c3p0.impl.NewPooledConnection.close(NewPooledConnection.java:646)
    at com.mchange.v2.c3p0.impl.NewPooledConnection.closeMaybeCheckedOut(NewPooledConnection.java:259)
    at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.destroyResource(C3P0PooledConnectionPool.java:619)
    at com.mchange.v2.resourcepool.BasicResourcePool$1DestroyResourceTask.run(BasicResourcePool.java:1024)
    at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:648)
[2013-12-06 11:06:21,716] [C3P0PooledConnectionPoolManager[identityToken->2tl0n98y1iwg7cbdzzq7a|719f1f]-HelperThread-#2] DEBUG - Successfully destroyed PooledConnection: com.mchange.v2.c3p0.impl.NewPooledConnection@484c6b

经过分析源码，得到的结论是这个异常可以无视掉。
相关源码如下(c3p0版本:c3p0-0.9.2.1)：
package com.mchange.v2.c3p0.impl;

import java.util.*;

import java.sql.*;

import javax.sql.*;

import com.mchange.v2.c3p0.*;

import com.mchange.v2.c3p0.stmt.*;

import com.mchange.v2.c3p0.util.*;

import com.mchange.v2.log.*;

import java.lang.reflect.Method;

import com.mchange.v2.lang.ObjectUtils;

import com.mchange.v2.sql.SqlUtils;

public final class NewPooledConnection extends AbstractC3P0PooledConnection{

    private final static MLogger logger = MLog.getLogger( NewPooledConnection.class );

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//  methods below must be called from sync'ed methods

    /*

     *  If a throwable cause is provided, the PooledConnection is known to be broken (cause is an invalidating exception)

     *  and this method will not throw any exceptions, even if some resource closes fail.

     *

     *  If cause is null, then we think the PooledConnection is healthy, and we will report (throw) an exception

     *  if resources unexpectedlay fail to close.

     */

    private void close( Throwable cause ) throws SQLException

    { close( cause, false ); }

    private void close( Throwable cause, boolean forced ) throws SQLException

    {

    assert Thread.holdsLock( this );

        if ( this.invalidatingException == null )

        {

            List closeExceptions = new LinkedList();

            // cleanup ResultSets

            cleanupResultSets( closeExceptions );

            // cleanup uncached Statements

        // System.err.println(this + ".close( ... ) -- uncachedActiveStatements: " + uncachedActiveStatements);

            cleanupUncachedStatements( closeExceptions );

            // cleanup cached Statements

            try

            { closeAllCachedStatements(); }

            catch ( SQLException e )

            { closeExceptions.add(e); }

        if ( forced )

        {

            // reset transaction state

            try { C3P0ImplUtils.resetTxnState( physicalConnection, forceIgnoreUnresolvedTransactions, autoCommitOnClose, false ); }

            catch (Exception e)

            {

                if (logger.isLoggable( MLevel.FINER ))

                logger.log( MLevel.FINER,

                        "Failed to reset the transaction state of  " + physicalConnection + "just prior to close(). " +

                        "Only relevant at all if this was a Connection being forced close()ed midtransaction.",

                        e );

            }

        }

            // cleanup physicalConnection

            try

            { physicalConnection.close(); }

            catch ( SQLException e )

            {

                if (logger.isLoggable( MLevel.FINER ))

                    logger.log( MLevel.FINER, "Failed to close physical Connection: " + physicalConnection, e );

                closeExceptions.add(e);

            }

            // update our state to bad status and closed, and log any exceptions

            if ( connection_status == ConnectionTester.CONNECTION_IS_OKAY )

                connection_status = ConnectionTester.CONNECTION_IS_INVALID;

            if ( cause == null )

            {

                this.invalidatingException = NORMAL_CLOSE_PLACEHOLDER;

                if ( Debug.DEBUG && logger.isLoggable( MLevel.FINEST ) )

                    logger.log( MLevel.FINEST, this + " closed by a client.", new Exception("DEBUG -- CLOSE BY CLIENT STACK TRACE") );//num:646 in the NewPooledConnection.java

/*

Note that:
1、This is not an exception, the new Exception is used merely to show execution path for debug purposes.
2、And yes, this is only a debug message (actually, FINEST is the lowest possible level in java.util.logging).
To wrap this up: ignore and tune your logging levels to skip these.

http://stackoverflow.com/questions/8403227/weird-error-close-by-client-stack-trace

既然成功了，干嘛还要丢异常出来？

这里就不得不说到两个商业开发的原则问题了。
第一，对上家传入数据严加过滤，对传出给下家的数据仔细检查。
第二，合理使用异常。
第一点其实很简单的。也就是模块化开发的一个思想问题。对自己的行为负责。前端返回的数据究竟是什么，需要进行校验。不合格的剔除或者是修正。合格的处理完后，在传出之前也要加以校验，是否合格

结合到 “合理使用异常” 这句话来说呢，就是说，需要抛出异常的时候，就抛出。不需要抛出的时候，就不抛出。对程序员来说，在必要的时候看到一串异常信息，是最合适的事情了。

http://www.zhixing123.cn/jsp/23305.html

 

*/

                 logCloseExceptions( null, closeExceptions );

                if (closeExceptions.size() > 0)

                    throw new SQLException("Some resources failed to close properly while closing " + this);

            }

            else

            {

                this.invalidatingException = cause;

                if (Debug.TRACE >= Debug.TRACE_MED)

                    logCloseExceptions( cause, closeExceptions );

                else

                    logCloseExceptions( cause, null );

            }

        }

    }
 
相关类：
/********************************************************************

 * This class generated by com.mchange.v2.debug.DebugGen

 * and will probably be overwritten by the same! Edit at

 * YOUR PERIL!!! Hahahahaha.

 ********************************************************************/

package com.mchange.v2.c3p0.impl;

import com.mchange.v2.debug.DebugConstants;

final class Debug implements DebugConstants

{

    final static boolean DEBUG = true;

    final static int     TRACE = TRACE_MAX;

    private Debug()

    {}

}

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.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
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.csharpcode .lnum { color: #606060; }

 
package com.mchange.v2.log;

import java.util.*;

public final class MLevel

{

    public final static MLevel ALL;

    public final static MLevel CONFIG;

    public final static MLevel FINE;

    public final static MLevel FINER;

    public final static MLevel FINEST;

    public final static MLevel INFO;

    public final static MLevel OFF;

    public final static MLevel SEVERE;

    public final static MLevel WARNING;

    private final static Map integersToMLevels;

    private final static Map namesToMLevels;

    public static MLevel fromIntValue(int intval)

    { return (MLevel) integersToMLevels.get( new Integer( intval ) ); }

    public static MLevel fromSeverity(String name)

    { return (MLevel) namesToMLevels.get( name ); }

    static

    {

    Class lvlClass;

    boolean jdk14api;  //not just jdk14 -- it is possible for the api to be present with older vms

    try

        {

        lvlClass = Class.forName( "java.util.logging.Level" );

        jdk14api = true;

        }

    catch (ClassNotFoundException e )

        {

        lvlClass = null;

        jdk14api = false;

        }

    MLevel all;

    MLevel config;

    MLevel fine;

    MLevel finer;

    MLevel finest;

    MLevel info;

    MLevel off;

    MLevel severe;

    MLevel warning;

    try

        {

        // numeric values match the intvalues from java.util.logging.Level

        all = new MLevel( (jdk14api ? lvlClass.getField("ALL").get(null) : null), Integer.MIN_VALUE, "ALL" );

        config = new MLevel( (jdk14api ? lvlClass.getField("CONFIG").get(null) : null), 700, "CONFIG" );

        fine = new MLevel( (jdk14api ? lvlClass.getField("FINE").get(null) : null), 500, "FINE" );

        finer = new MLevel( (jdk14api ? lvlClass.getField("FINER").get(null) : null), 400, "FINER" );

        finest = new MLevel( (jdk14api ? lvlClass.getField("FINEST").get(null) : null), 300, "FINEST" );

        info = new MLevel( (jdk14api ? lvlClass.getField("INFO").get(null) : null), 800, "INFO" );

        off = new MLevel( (jdk14api ? lvlClass.getField("OFF").get(null) : null), Integer.MAX_VALUE, "OFF" );

        severe = new MLevel( (jdk14api ? lvlClass.getField("SEVERE").get(null) : null), 900, "SEVERE" );

        warning = new MLevel( (jdk14api ? lvlClass.getField("WARNING").get(null) : null), 1000, "WARNING" );

        }

    catch ( Exception e )

        {

        e.printStackTrace();

        throw new InternalError("Huh? java.util.logging.Level is here, but not its expected public fields?");

        }

    ALL = all;

    CONFIG = config;

    FINE = fine;

    FINER = finer;

    FINEST = finest;

    INFO = info;

    OFF = off;

    SEVERE = severe;

    WARNING = warning;

    Map tmp = new HashMap();

    tmp.put( new Integer(all.intValue()), all);

    tmp.put( new Integer(config.intValue()), config);

    tmp.put( new Integer(fine.intValue()), fine);

    tmp.put( new Integer(finer.intValue()), finer);

    tmp.put( new Integer(finest.intValue()), finest);

    tmp.put( new Integer(info.intValue()), info);

    tmp.put( new Integer(off.intValue()), off);

    tmp.put( new Integer(severe.intValue()), severe);

    tmp.put( new Integer(warning.intValue()), warning);

    integersToMLevels = Collections.unmodifiableMap( tmp );

    tmp = new HashMap();

    tmp.put( all.getSeverity(), all);

    tmp.put( config.getSeverity(), config);

    tmp.put( fine.getSeverity(), fine);

    tmp.put( finer.getSeverity(), finer);

    tmp.put( finest.getSeverity(), finest);

    tmp.put( info.getSeverity(), info);

    tmp.put( off.getSeverity(), off);

    tmp.put( severe.getSeverity(), severe);

    tmp.put( warning.getSeverity(), warning);

    namesToMLevels = Collections.unmodifiableMap( tmp );

    }

    Object level;

    int    intval;

    String lvlstring;

    public int intValue()

    { return intval; }

    public Object asJdk14Level()

    { return level; }

    public String getSeverity()

    { return lvlstring; }

    public String toString()

    { return this.getClass().getName() + this.getLineHeader(); }

    public String getLineHeader()

    { return "[" + lvlstring + ']';}

    public boolean isLoggable( MLevel filterLevel )

    { return this.intval >= filterLevel.intval; }

    private MLevel(Object level, int intval, String lvlstring)

    {

    this.level = level;

    this.intval = intval;

    this.lvlstring = lvlstring;

    }

}

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.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
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package java.util.logging;

import java.util.*;

import java.security.*;

import java.lang.ref.WeakReference;

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font-family: consolas, "Courier New", courier, monospace;
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.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
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.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
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private static final int offValue = Level.OFF.intValue();
    private volatile int levelValue;  // current effective level value
    /** * Check if a message of the given level would actually be logged

 * by this logger. This check is based on the Loggers effective level,

 * which may be inherited from its parent. *

 * @param level a message logging level

 * @return true if the given message level is currently being logged. */

    public boolean isLoggable(Level level) {

    if (level.intValue() < levelValue || levelValue == offValue) {

        return false;    }    return true;    }

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.csharpcode .rem { color: #008000; }
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.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
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.csharpcode .lnum { color: #606060; }
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.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
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