Java中常用的數(shù)據(jù)庫(kù)連接池_動(dòng)力節(jié)點(diǎn)Java學(xué)院整理

更新時(shí)間：2017年08月03日 10:25:49 投稿：mrr

數(shù)據(jù)庫(kù)連接池負(fù)責(zé)分配、管理和釋放數(shù)據(jù)庫(kù)連接，它允許應(yīng)用程序重復(fù)使用一個(gè)現(xiàn)有的數(shù)據(jù)庫(kù)連接，而不是再重新建立一個(gè)；釋放空閑時(shí)間超過(guò)最大空閑時(shí)間的數(shù)據(jù)庫(kù)連接來(lái)避免因?yàn)闆](méi)有釋放數(shù)據(jù)庫(kù)連接而引起的數(shù)據(jù)庫(kù)連接遺漏

定義

數(shù)據(jù)庫(kù)連接是一種關(guān)鍵的有限的昂貴的資源，這一點(diǎn)在多用戶的網(wǎng)頁(yè)應(yīng)用程序中體現(xiàn)得尤為突出。對(duì)數(shù)據(jù)庫(kù)連接的管理能顯著影響到整個(gè)應(yīng)用程序的伸縮性和健壯性，影響到程序的性能指標(biāo)。數(shù)據(jù)庫(kù)連接池正是針對(duì)這個(gè)問(wèn)題提出來(lái)的。

參考資料

DBCP

下載地址：http://commons.apache.org/proper/commons-dbcp/download_dbcp.cgi

相關(guān)屬性說(shuō)明: http://commons.apache.org/proper/commons-dbcp/configuration.html

c3p0

下載地址：http://sourceforge.net/projects/c3p0/

相關(guān)屬性說(shuō)明: http://www.mchange.com/projects/c3p0/

Druid

下載地址：http://repo1.maven.org/maven2/com/alibaba/druid/

相關(guān)屬性說(shuō)明: https://github.com/alibaba/druid/wiki

Proxool

下載地址：http://proxool.sourceforge.net/

相關(guān)屬性說(shuō)明：https://github.com/proxool/proxool

2. 數(shù)據(jù)庫(kù)連接池

2.1 原理

連接池基本的思想是在系統(tǒng)初始化的時(shí)候，將數(shù)據(jù)庫(kù)連接作為對(duì)象存儲(chǔ)在內(nèi)存中，當(dāng)用戶需要訪問(wèn)數(shù)據(jù)庫(kù)時(shí)，并非建立一個(gè)新的連接，而是從連接池中取出一個(gè)已建立的空閑連接對(duì)象。使用完畢后，用戶也并非將連接關(guān)閉，而是將連接放回連接池中，以供下一個(gè)請(qǐng)求訪問(wèn)使用。而連接的建立、斷開(kāi)都由連接池自身來(lái)管理。同時(shí)，還可以通過(guò)設(shè)置連接池的參數(shù)來(lái)控制連接池中的初始連接數(shù)、連接的上下限數(shù)以及每個(gè)連接的最大使用次數(shù)、最大空閑時(shí)間等等，也可以通過(guò)其自身的管理機(jī)制來(lái)監(jiān)視數(shù)據(jù)庫(kù)連接的數(shù)量、使用情況等。

2.2 常見(jiàn)數(shù)據(jù)庫(kù)連接池及其特點(diǎn)

在Java中開(kāi)源的常用的數(shù)據(jù)庫(kù)連接池有以下幾種：

1）DBCP

DBCP是一個(gè)依賴Jakarta commons-pool對(duì)象池機(jī)制的數(shù)據(jù)庫(kù)連接池.DBCP可以直接的在應(yīng)用程序中使用，Tomcat的數(shù)據(jù)源使用的就是DBCP。

2）c3p0

c3p0是一個(gè)開(kāi)放源代碼的JDBC連接池，它在lib目錄中與Hibernate一起發(fā)布,包括了實(shí)現(xiàn)jdbc3和jdbc2擴(kuò)展規(guī)范說(shuō)明的Connection 和Statement 池的DataSources 對(duì)象。

3）Druid

阿里出品，淘寶和支付寶專用數(shù)據(jù)庫(kù)連接池，但它不僅僅是一個(gè)數(shù)據(jù)庫(kù)連接池，它還包含一個(gè)ProxyDriver，一系列內(nèi)置的JDBC組件庫(kù)，一個(gè)SQL Parser。支持所有JDBC兼容的數(shù)據(jù)庫(kù)，包括Oracle、MySql、Derby、Postgresql、SQL Server、H2等等。
Druid針對(duì)Oracle和MySql做了特別優(yōu)化，比如Oracle的PS Cache內(nèi)存占用優(yōu)化，MySql的ping檢測(cè)優(yōu)化。

Druid提供了MySql、Oracle、Postgresql、SQL-92的SQL的完整支持，這是一個(gè)手寫(xiě)的高性能SQL Parser，支持Visitor模式，使得分析SQL的抽象語(yǔ)法樹(shù)很方便。

簡(jiǎn)單SQL語(yǔ)句用時(shí)10微秒以內(nèi)，復(fù)雜SQL用時(shí)30微秒。

通過(guò)Druid提供的SQL Parser可以在JDBC層攔截SQL做相應(yīng)處理，比如說(shuō)分庫(kù)分表、審計(jì)等。Druid防御SQL注入攻擊的WallFilter就是通過(guò)Druid的SQL Parser分析語(yǔ)義實(shí)現(xiàn)的。

4) proxool

Proxool是一種Java數(shù)據(jù)庫(kù)連接池技術(shù)。sourceforge下的一個(gè)開(kāi)源項(xiàng)目,這個(gè)項(xiàng)目提供一個(gè)健壯、易用的連接池，最為關(guān)鍵的是這個(gè)連接池提供監(jiān)控的功能，方便易用，便于發(fā)現(xiàn)連接泄漏的情況。目前是和DBCP以及C3P0一起，最為常見(jiàn)的三種JDBC連接池技術(shù)。目前，Hibernate官方宣布由于Bug太多不再支持DBCP，而推薦使用 Proxool或C3P0。

3. 主要配置說(shuō)明

連接池配置大體可以分為基本配置、關(guān)鍵配置、性能配置等主要配置,缺少proxool相關(guān)配置。

3.1 基本配置

基本配置是指連接池進(jìn)行數(shù)據(jù)庫(kù)連接的四個(gè)基本必需配置：

傳遞給JDBC驅(qū)動(dòng)的用于連接數(shù)據(jù)庫(kù)的用戶名、密碼、URL以及驅(qū)動(dòng)類名。

	DBCP	c3p0	Druid
用戶名	username	user	username
密碼	password	password	password
URL	url	jdbcUrl	jdbcUrl
驅(qū)動(dòng)類名	driverClassName	driverClass	driverClassName

注:在Druid連接池的配置中，driverClassName可配可不配，如果不配置會(huì)根據(jù)url自動(dòng)識(shí)別dbType(數(shù)據(jù)庫(kù)類型)，然后選擇相應(yīng)的driverClassName。

3.2 關(guān)鍵配置

為了發(fā)揮數(shù)據(jù)庫(kù)連接池的作用，在初始化時(shí)將創(chuàng)建一定數(shù)量的數(shù)據(jù)庫(kù)連接放到連接池中，這些數(shù)據(jù)庫(kù)連接的數(shù)量是由最小數(shù)據(jù)庫(kù)連接數(shù)來(lái)設(shè)定的。無(wú)論這些數(shù)據(jù)庫(kù)連接是否被使用，連接池都將一直保證至少擁有這么多的連接數(shù)量。連接池的最大數(shù)據(jù)庫(kù)連接數(shù)量限定了這個(gè)連接池能占有的最大連接數(shù)，當(dāng)應(yīng)用程序向連接池請(qǐng)求的連接數(shù)超過(guò)最大連接數(shù)量時(shí)，這些請(qǐng)求將被加入到等待隊(duì)列中。

最小連接數(shù):

是數(shù)據(jù)庫(kù)一直保持的數(shù)據(jù)庫(kù)連接數(shù)，所以如果應(yīng)用程序?qū)?shù)據(jù)庫(kù)連接的使用量不大，將有大量的數(shù)據(jù)庫(kù)資源被浪費(fèi)。

初始化連接數(shù)：

連接池啟動(dòng)時(shí)創(chuàng)建的初始化數(shù)據(jù)庫(kù)連接數(shù)量。

最大連接數(shù)：

是連接池能申請(qǐng)的最大連接數(shù)，如果數(shù)據(jù)庫(kù)連接請(qǐng)求超過(guò)此數(shù)，后面的數(shù)據(jù)庫(kù)連接請(qǐng)求被加入到等待隊(duì)列中。

最大等待時(shí)間：

當(dāng)沒(méi)有可用連接時(shí)，連接池等待連接被歸還的最大時(shí)間，超過(guò)時(shí)間則拋出異常，可設(shè)置參數(shù)為0或者負(fù)數(shù)使得無(wú)限等待(根據(jù)不同連接池配置)。

	DBCP	c3p0	Druid
最小連接數(shù)	minIdle(0)	minPoolSize(3)	minIdle(0)
初始化連接數(shù)	initialSize(0)	initialPoolSize(3)	initialSize(0)
最大連接數(shù)	maxTotal(8)	maxPoolSize(15)	maxActive(8)
最大等待時(shí)間	maxWaitMillis(毫秒)	maxIdleTime(0秒)	maxWait(毫秒)

注1:在DBCP連接池的配置中，還有一個(gè)maxIdle的屬性，表示最大空閑連接數(shù)，超過(guò)的空閑連接將被釋放，默認(rèn)值為8。對(duì)應(yīng)的該屬性在Druid連接池已不再使用，配置了也沒(méi)有效果，c3p0連接池則沒(méi)有對(duì)應(yīng)的屬性。

注2：數(shù)據(jù)庫(kù)連接池在初始化的時(shí)候會(huì)創(chuàng)建initialSize個(gè)連接，當(dāng)有數(shù)據(jù)庫(kù)操作時(shí)，會(huì)從池中取出一個(gè)連接。如果當(dāng)前池中正在使用的連接數(shù)等于maxActive，則會(huì)等待一段時(shí)間，等待其他操作釋放掉某一個(gè)連接，如果這個(gè)等待時(shí)間超過(guò)了maxWait，則會(huì)報(bào)錯(cuò)；如果當(dāng)前正在使用的連接數(shù)沒(méi)有達(dá)到maxActive，則判斷當(dāng)前是否空閑連接，如果有則直接使用空閑連接，如果沒(méi)有則新建立一個(gè)連接。在連接使用完畢后，不是將其物理連接關(guān)閉，而是將其放入池中等待其他操作復(fù)用。

3.3 性能配置

預(yù)緩存設(shè)置:

即是PSCache，PSCache對(duì)支持游標(biāo)的數(shù)據(jù)庫(kù)性能提升巨大，比如說(shuō)oracle。JDBC的標(biāo)準(zhǔn)參數(shù)，用以控制數(shù)據(jù)源內(nèi)加載的PreparedStatements數(shù)量。但由于預(yù)緩存的statements屬于單個(gè)connection而不是整個(gè)連接池，所以設(shè)置這個(gè)參數(shù)需要考慮到多方面的因素。

單個(gè)連接擁有的最大緩存數(shù)：要啟用PSCache，必須配置大于0，當(dāng)大于0時(shí)，poolPreparedStatements自動(dòng)觸發(fā)修改為true。在Druid中，不會(huì)存在Oracle下PSCache占用內(nèi)存過(guò)多的問(wèn)題，可以把這個(gè)數(shù)值配置大一些，比如說(shuō)100

	DBCP	c3p0	Druid
開(kāi)啟緩存功能	poolPreparedStatements	maxStatements	poolPreparedStatements
單個(gè)連接擁有的最大緩存數(shù)	maxOpenPrepared- Statements	maxStatementsPer- Connection	maxOpenPrepared- Statements

連接有效性檢測(cè)設(shè)置:

連接池內(nèi)部有機(jī)制判斷，如果當(dāng)前的總的連接數(shù)少于miniIdle，則會(huì)建立新的空閑連接，以保證連接數(shù)得到miniIdle。如果當(dāng)前連接池中某個(gè)連接在空閑了timeBetweenEvictionRunsMillis時(shí)間后任然沒(méi)有使用，則被物理性的關(guān)閉掉。有些數(shù)據(jù)庫(kù)連接的時(shí)候有超時(shí)限制（mysql連接在8小時(shí)后斷開(kāi)），或者由于網(wǎng)絡(luò)中斷等原因，連接池的連接會(huì)出現(xiàn)失效的情況，這時(shí)候設(shè)置一個(gè)testWhileIdle參數(shù)為true，可以保證連接池內(nèi)部定時(shí)檢測(cè)連接的可用性，不可用的連接會(huì)被拋棄或者重建，最大情況的保證從連接池中得到的Connection對(duì)象是可用的。當(dāng)然，為了保證絕對(duì)的可用性，你也可以使用testOnBorrow為true（即在獲取Connection對(duì)象時(shí)檢測(cè)其可用性），不過(guò)這樣會(huì)影響性能。

	DBCP	c3p0	Druid
申請(qǐng)連接檢測(cè)	testOnBorrow	testConnectionOnCheckin	testOnBorrow
是否超時(shí)檢測(cè)	testWhileIdle		testWhileIdle
空閑時(shí)間	timeBetweenEvictionRunsMillis	idleConnectionTestPeriod	timeBetweenEvictionRunsMillis
校驗(yàn)用sql語(yǔ)句	validationQuery	preferredTestQuery	validationQuery
歸還連接檢測(cè)	testOnReturn	testConnectionOnCheckout	testOnReturn

超時(shí)連接關(guān)閉設(shè)置:

removeAbandoned參數(shù)，用來(lái)檢測(cè)到當(dāng)前使用的連接是否發(fā)生了連接泄露，所以在代碼內(nèi)部就假定如果一個(gè)連接建立連接的時(shí)間很長(zhǎng)，則將其認(rèn)定為泄露，繼而強(qiáng)制將其關(guān)閉掉。

	DBCP	c3p0	Druid
是否超時(shí)關(guān)閉連接	removeAbandoned	breakAfterAcquireFailure	removeAbandoned
超時(shí)時(shí)間	removeAbandonedTimeout	checkoutTimeout	removeAbandonedTimeout
是否記錄日志	logAbandoned		logAbandoned

c3p0重連設(shè)置:

設(shè)置獲取連接失敗后，是否重新連接以及間隔時(shí)間。

	DBCP	c3p0	Druid
重連次數(shù)		acquireRetryAttempts
間隔時(shí)間		acquireRetryDelay

4. 配置詳解

4.1 DBCP 屬性說(shuō)明表

屬性(Parameter)	默認(rèn)值(Default)	描述(Description)
username		傳遞給JDBC驅(qū)動(dòng)的用于建立連接的用戶名(The connection username to be passed to our JDBC driver to establish a connection.)
password		傳遞給JDBC驅(qū)動(dòng)的用于建立連接的密碼(The connection password to be passed to our JDBC driver to establish a connection.)
url		傳遞給JDBC驅(qū)動(dòng)的用于建立連接的URL(The connection URL to be passed to our JDBC driver to establish a connection.)
driverClassName		使用的JDBC驅(qū)動(dòng)的完整有效的java 類名(The fully qualified Java class name of the JDBC driver to be used.)
defaultAutoCommit	driver default	連接池創(chuàng)建的連接的默認(rèn)的auto-commit狀態(tài),沒(méi)有設(shè)置則不會(huì)自動(dòng)提交(The default auto-commit state of connections created by this pool. If not set then the setAutoCommit method will not be called.)
initialSize	0	初始化連接:連接池啟動(dòng)時(shí)創(chuàng)建的初始化連接數(shù)量(The initial number of connections that are created when the pool is started.
maxTotal	8	最大活動(dòng)連接:連接池在同一時(shí)間能夠分配的最大活動(dòng)連接的數(shù)量, 如果設(shè)置為非正數(shù)則表示不限制(The maximum number of active connections that can be allocated from this pool at the same time, or negative for no limit.)
maxIdle	8	最大空閑連接:連接池中容許保持空閑狀態(tài)的最大連接數(shù)量,超過(guò)的空閑連接將被釋放,如果設(shè)置為負(fù)數(shù)表示不限制(The maximum number of connections that can remain idle in the pool, without extra ones being released, or negative for no limit.)
minIdle	0	最小空閑連接:連接池中容許保持空閑狀態(tài)的最小連接數(shù)量,負(fù)數(shù)表示沒(méi)有現(xiàn)在(The maximum number of connections that can remain idle in the pool, without extra ones being released, or negative for no limit.)
注意：如果在某些負(fù)載比較大的系統(tǒng)中將maxIdel設(shè)置過(guò)小時(shí)，很可能會(huì)出現(xiàn)連接關(guān)閉的同時(shí)新連接馬上打開(kāi)的情況.這是由于關(guān)閉連接的線程比打開(kāi)的快導(dǎo)致的.所以，對(duì)于這種系統(tǒng)中,maxIdle的設(shè)定值是不同的但是通常首選默認(rèn)值
(NOTE: If maxIdle is set too low on heavily loaded systems it is possible you will see connections being closed and almost immediately new connections being opened. This is a result of the active threads momentarily closing connections faster than they are opening them, causing the number of idle connections to rise above maxIdle. The best value for maxIdle for heavily loaded system will vary but the default is a good starting point.)
maxWaitMillis	indefinitely	最大等待時(shí)間:當(dāng)沒(méi)有可用連接時(shí),連接池等待連接被歸還的最大時(shí)間(以毫秒計(jì)數(shù)),超過(guò)時(shí)間則拋出異常,如果設(shè)置為-1表示無(wú)限等待(The maximum number of milliseconds that the pool will wait (when there are no available connections) for a connection to be returned before throwing an exception, or -1 to wait indefinitely.)
validationQuery		SQL查詢,用來(lái)驗(yàn)證從連接池取出的連接,在將連接返回給調(diào)用者之前.如果指定,則查詢必須是一個(gè)SQL SELECT并且必須返回至少一行記錄(The SQL query that will be used to validate connections from this pool before returning them to the caller. If specified, this query MUST be an SQL SELECT statement that returns at least one row. If not specified, connections will be validation by calling the isValid() method.)
testOnCreate	false	指明是否在建立連接之后進(jìn)行驗(yàn)證,如果驗(yàn)證失敗,則嘗試重新建立連接(The indication of whether objects will be validated after creation. If the object fails to validate, the borrow attempt that triggered the object creation will fail.)
testOnBorrow	true	指明是否在從池中取出連接前進(jìn)行檢驗(yàn),如果檢驗(yàn)失敗,則從池中去除連接并嘗試取出另一個(gè). 注意:設(shè)置為true后如果要生效,validationQuery參數(shù)必須設(shè)置為非空字符串(The indication of whether objects will be validated before being borrowed from the pool. If the object fails to validate, it will be dropped from the pool, and we will attempt to borrow another.)
testOnReturn	false	指明是否在歸還到池中前進(jìn)行檢驗(yàn)(The indication of whether objects will be validated before being returned to the pool.)
testWhileIdle	false	指明連接是否被空閑連接回收器(如果有)進(jìn)行檢驗(yàn).如果檢測(cè)失敗,則連接將被從池中去除. 注意:設(shè)置為true后如果要生效,validationQuery參數(shù)必須設(shè)置為非空字符串(The indication of whether objects will be validated by the idle object evictor (if any). If an object fails to validate, it will be dropped from the pool.)
timeBetweenEviction- RunsMillis	-1	在空閑連接回收器線程運(yùn)行期間休眠的時(shí)間值,以毫秒為單位.如果設(shè)置為非正數(shù),則不運(yùn)行空閑連接回收器線程(The number of milliseconds to sleep between runs of the idle object evictor thread. When non-positive, no idle object evictor thread will be run.)
numTestsPerEvictionRun	3	在每次空閑連接回收器線程(如果有)運(yùn)行時(shí)檢查的連接數(shù)量(The number of objects to examine during each run of the idle object evictor thread (if any).)
minEvictableIdleTime-Millis	*10006030*	連接在池中保持空閑而不被空閑連接回收器線程(如果有)回收的最小時(shí)間值，單位毫秒(The minimum amount of time an object may sit idle in the pool before it is eligable for eviction by the idle object evictor (if any).)
softMiniEvictableIdle- TimeMillis	-1	說(shuō)明(The minimum amount of time a connection may sit idle in the pool before it is eligible for eviction by the idle connection evictor, with the extra condition that at least "minIdle" connections remain in the pool. When miniEvictableIdleTimeMillis is set to a positive value, miniEvictableIdleTimeMillis is examined first by the idle connection evictor - i.e. when idle connections are visited by the evictor, idle time is first compared against miniEvictableIdleTimeMillis (without considering the number of idle connections in the pool) and then against softMinEvictableIdleTimeMillis, including the minIdle constraint.)
maxConnLifetimeMillis	-1	說(shuō)明(The maximum lifetime in milliseconds of a connection. After this time is exceeded the connection will fail the next activation, passivation or validation test. A value of zero or less means the connection has an infinite lifetime.)
logExpiredConnections	true	說(shuō)明(Flag to log a message indicating that a connection is being closed by the pool due to maxConnLifetimeMillis exceeded. Set this property to false to suppress expired connection logging that is turned on by default.
connectionInitSqls	null	說(shuō)明(A Collection of SQL statements that will be used to initialize physical connections when they are first created. These statements are executed only once - when the configured connection factory creates the connection.)
info	true	說(shuō)明(True means that borrowObject returns the most recently used ("last in") connection in the pool (if there are idle connections available). False means that the pool behaves as a FIFO queue - connections are taken from the idle instance pool in the order that they are returned to the pool.)
poolPreparedState-ments	false	開(kāi)啟池的prepared statement 池功能(Enable prepared statement pooling for this pool.)
maxOpenPreparedState-ments	unlimited	statement池能夠同時(shí)分配的打開(kāi)的statements的最大數(shù)量, 如果設(shè)置為0表示不限制(The maximum number of open statements that can be allocated from the statement pool at the same time, or negative for no limit.)
NOTE - Make sure your connection has some resources left for the other statements. Pooling PreparedStatements may keep their cursors open in the database, causing a connection to run out of cursors, especially if maxOpenPreparedStatements is left at the default (unlimited) and an application opens a large number of different PreparedStatements per connection. To avoid this problem, maxOpenPreparedStatements should be set to a value less than the maximum number of cursors that can be open on a Connection.
accessToUnderlyingConnectionAllowed	false	控制PoolGuard是否容許獲取底層連接(Controls if the PoolGuard allows access to the underlying connection.) 默認(rèn)false不開(kāi)啟,這是一個(gè)有潛在危險(xiǎn)的功能, 不適當(dāng)?shù)木幋a會(huì)造成傷害.(關(guān)閉底層連接或者在守護(hù)連接已經(jīng)關(guān)閉的情況下繼續(xù)使用它).請(qǐng)謹(jǐn)慎使用,并且僅當(dāng)需要直接訪問(wèn)驅(qū)動(dòng)的特定功能時(shí)使用.注意: 不要關(guān)閉底層連接, 只能關(guān)閉前面的那個(gè). Default is false, it is a potential dangerous operation and misbehaving programs can do harmful things. (closing the underlying or continue using it when the guarded connection is already closed) Be careful and only use when you need direct access to driver specific extensions. NOTE: Do not close the underlying connection, only the original one.
removeAbandoned	false	標(biāo)記是否刪除泄露的連接,如果他們超過(guò)了removeAbandonedTimout的限制.如果設(shè)置為true, 連接被認(rèn)為是被泄露并且可以被刪除,如果空閑時(shí)間超過(guò)removeAbandonedTimeout. 設(shè)置為true可以為寫(xiě)法糟糕的沒(méi)有關(guān)閉連接的程序修復(fù)數(shù)據(jù)庫(kù)連接. (Flags to remove abandoned connections if they exceed the removeAbandonedTimout. A connection is considered abandoned and eligible for removal if it has not been used for longer than removeAbandonedTimeout. Setting one or both of these to true can recover db connections from poorly written applications which fail to close connections.)
removeAbandonedTimeout	300	泄露的連接可以被刪除的超時(shí)值, 單位秒(Timeout in seconds before an abandoned connection can be removed.)
logAbandoned	false	標(biāo)記當(dāng)Statement或連接被泄露時(shí)是否打印程序的stack traces日志。被泄露的Statements和連接的日志添加在每個(gè)連接打開(kāi)或者生成新的Statement,因?yàn)樾枰蓅tack trace。(Flag to log stack traces for application code which abandoned a Statement or Connection. Logging of abandoned Statements and Connections adds overhead for every Connection open or new Statement because a stack trace has to be generated.)
abandonedUsageTracking	false	如果為true, 那么連接池會(huì)記錄每個(gè)方法調(diào)用時(shí)候的堆棧信息以及廢棄連接的調(diào)試信息(If true, the connection pool records a stack trace every time a method is called on a pooled connection and retains the most recent stack trace to aid debugging of abandoned connections. There is significant overhead added by setting this to true.)

注:如果開(kāi)啟"removeAbandoned",那么連接在被認(rèn)為泄露時(shí)可能被池回收. 這個(gè)機(jī)制在(getNumIdle() < 2)and (getNumActive() > getMaxActive() - 3)時(shí)被觸發(fā). 舉例當(dāng)maxActive=20, 活動(dòng)連接為18,空閑連接為1時(shí)可以觸發(fā)"removeAbandoned".但是活動(dòng)連接只有在沒(méi)有被使用的時(shí)間超過(guò)"removeAbandonedTimeout"時(shí)才被刪除,默認(rèn)300秒.在resultset中游歷不被計(jì)算為被使用.

If you have enabled removeAbandonedOnMaintenance or removeAbandonedOnBorrow then it is possible that a connection is reclaimed by the pool because it is considered to be abandoned. This mechanism is triggered when (getNumIdle() < 2) and (getNumActive() > getMaxTotal() - 3) and removeAbandonedOnBorrow is true; or after eviction finishes and removeAbandonedOnMaintenance is true. For example, maxTotal=20 and 18 active connections and 1 idle connection would trigger removeAbandonedOnBorrow, but only the active connections that aren't used for more then "removeAbandonedTimeout" seconds are removed (default 300 sec). Traversing a resultset doesn't count as being used. Creating a Statement, PreparedStatement or CallableStatement or using one of these to execute a query (using one of the execute methods) resets the lastUsed property of the parent connection.

4.2 C3P0 屬性說(shuō)明表

屬性(Parameter)	默認(rèn)值(Default)	描述(Description)
user		同DBCP中的username屬性
password		同DBCP中的password屬性
jdbcUrl		同DBCP中的jdbcUrl屬性
driverClass		同DBCP中的driverClass屬性
autoCommitOnClose	false	默認(rèn)值false表示回滾任何未提交的任務(wù),設(shè)置為true則全部提交,而不是在關(guān)閉連接之前回滾 (C3P0's default policy is to rollback any uncommitted, pending work. Setting autoCommitOnClose to true causes uncommitted pending work to be committed, rather than rolled back on Connection close.) *參見(jiàn)DBCP中的defaultAutoCommit屬性
initialPoolSize	3	初始化連接:連接池啟動(dòng)時(shí)創(chuàng)建的初始化連接數(shù)量(The initial number of connections that are created when the pool is started. *參見(jiàn)DBCP中的initialSize屬性
maxPoolSize	15	連接池中保留的最大連接數(shù)(Maximum number of Connections a pool will maintain at any given time.) *參見(jiàn)DBCP中的maxIdle屬性
minPoolSize	3	連接池中保留的最小連接數(shù)(Minimum number of Connections a pool will maintain at any given time.) *參見(jiàn)DBCP中的maxIdle屬性
maxIdleTime	0	最大等待時(shí)間:當(dāng)沒(méi)有可用連接時(shí),連接池等待連接被歸還的最大時(shí)間(以秒計(jì)數(shù)),超過(guò)時(shí)間則拋出異常,如果設(shè)置為0表示無(wú)限等待(Seconds a Connection can remain pooled but unused before being discarded. Zero means idle connections never expire.) *參見(jiàn)DBCP中maxWaitMillis 屬性
preferredTestQuery	null	定義所有連接測(cè)試都執(zhí)行的測(cè)試語(yǔ)句。在使用連接測(cè)試的情況下這個(gè)一顯著提高測(cè)試速度。注意：測(cè)試的表必須在初始數(shù)據(jù)源的時(shí)候就存在。(Defines the query that will be executed for all connection tests, if the default ConnectionTester (or some other implementation of QueryConnectionTester, or better yet FullQueryConnectionTester) is being used. Defining a preferredTestQuery that will execute quickly in your database may dramatically speed up Connection tests.)
testConnectionOn- Checkin	false	如果設(shè)為true那么在取得連接的同時(shí)將校驗(yàn)連接的有效性。(If true, an operation will be performed asynchronously at every connection checkin to verify that the connection is valid. Use in combination with idleConnectionTestPeriod for quite reliable, always asynchronous Connection testing.) *參見(jiàn)DBCP中的testOnBorrow屬性
testConnectionOn- Checkout	false	如果設(shè)為true那么在每個(gè)connection提交的時(shí)候都將校驗(yàn)其有效性,但是要確保配置的preferredTestQuery的有效性(If true, an operation will be performed at every connection checkout to verify that the connection is valid. Be sure to set an efficient preferredTestQuery or automaticTestTable if you set this to true.) *參見(jiàn)DBCP中的testOnBorrow屬性
idleConnectionTest- Period	0	如果設(shè)置大于0,表示過(guò)了多少秒檢查一次空閑連接,結(jié)合testConnectionOnCheckin以及testConnectionOnCheckout使用(If this is a number greater than 0, c3p0 will test all idle, pooled but unchecked-out connections, every this number of seconds.)
acquireRetryAttempts	30	定義在從數(shù)據(jù)庫(kù)獲取新連接失敗后重復(fù)嘗試的次數(shù), 如果小于0則表示無(wú)限制的連接。(Defines how many times c3p0 will try to acquire a new Connection from the database before giving up. If this value is less than or equal to zero, c3p0 will keep trying to fetch a Connection indefinitely.)
acquireRetryDelay	1000	兩次連接中的間隔時(shí)間，單位毫秒。(Milliseconds, time c3p0 will wait between acquire attempts.)
breakAfterAcquire- Failure	false	獲取連接失敗將會(huì)引起所有等待連接池來(lái)獲取連接的線程拋出異常。但是數(shù)據(jù)源仍有效保留，并在下次調(diào)用 getConnection() 的時(shí)候繼續(xù)嘗試獲取連接。如果為 true，那么在嘗試獲取連接失敗后該數(shù)據(jù)源將聲明已斷開(kāi)并永久關(guān)閉。(If true, a pooled DataSource will declare itself broken and be permanently closed if a Connection cannot be obtained from the database after making acquireRetryAttempts to acquire one. If false, failure to obtain a Connection will cause all Threads waiting for the pool to acquire a Connection to throw an Exception, but the DataSource will remain valid, and will attempt to acquire again following a call to getConnection().)
checkoutTimeout	0	當(dāng)連接池用完時(shí)客戶端調(diào)用 getConnection()后等待獲取新連接的時(shí)間，潮濕后將拋出SQLException，如設(shè)為0，則為無(wú)限期等待。單位毫秒。(The number of milliseconds a client calling getConnection() will wait for a Connection to be checked-in or acquired when the pool is exhausted. Zero means wait indefinitely. Setting any positive value will cause the getConnection() call to time-out and break with an SQLException after the specified number of milliseconds.)
maxStatements	0	控制數(shù)據(jù)源內(nèi)加載的PreparedStatements數(shù)量(Enable prepared statement pooling for this pool.)
maxStatementsPer- Connection	0	定義了連接池內(nèi)單個(gè)連接所擁有的最大緩存statements數(shù)(The maximum number of open statements that can be allocated from the statement pool at the same time, or negative for no limit.)

4.3 DRUID 屬性說(shuō)明表

屬性(Parameter)	默認(rèn)值(Default)	描述(Description)
username		連接數(shù)據(jù)庫(kù)的用戶名
password		連接數(shù)據(jù)庫(kù)的密碼
jdbcUrl		同DBCP中的jdbcUrl屬性
driverClassName	根據(jù)url自動(dòng)識(shí)別	這一項(xiàng)可配可不配，如果不配置druid會(huì)根據(jù)url自動(dòng)識(shí)別dbType，然后選擇相應(yīng)的driverClassName
initialSize	0	初始化時(shí)建立物理連接的個(gè)數(shù)。初始化發(fā)生在顯示調(diào)用init方法，或者第一次getConnection時(shí) *參見(jiàn)DBCP中的initialSize屬性
maxActive	8	最大連接池?cái)?shù)量(Maximum number of Connections a pool will maintain at any given time.) *參見(jiàn)DBCP中的maxTotal屬性
maxIdle	8	已經(jīng)不再使用，配置了也沒(méi)效果 *參見(jiàn)DBCP中的maxIdle屬性
minIdle		最小連接池?cái)?shù)量
maxWait		獲取連接時(shí)最大等待時(shí)間，單位毫秒。配置了maxWait之后，缺省啟用公平鎖，并發(fā)效率會(huì)有所下降，如果需要可以通過(guò)配置useUnfairLock屬性為true使用非公平鎖。
poolPreparedState- ments	false	是否緩存preparedStatement，也就是PSCache。PSCache對(duì)支持游標(biāo)的數(shù)據(jù)庫(kù)性能提升巨大，比如說(shuō)oracle。
maxOpenPrepared- Statements	-1	要啟用PSCache，必須配置大于0，當(dāng)大于0時(shí)，poolPreparedStatements自動(dòng)觸發(fā)修改為true。在Druid中，不會(huì)存在Oracle下PSCache占用內(nèi)存過(guò)多的問(wèn)題，可以把這個(gè)數(shù)值配置大一些，比如說(shuō)100
testOnBorrow	true	申請(qǐng)連接時(shí)執(zhí)行validationQuery檢測(cè)連接是否有效，做了這個(gè)配置會(huì)降低性能。
testOnReturn	false	歸還連接時(shí)執(zhí)行validationQuery檢測(cè)連接是否有效，做了這個(gè)配置會(huì)降低性能
testWhileIdle	false	建議配置為true，不影響性能，并且保證安全性。申請(qǐng)連接的時(shí)候檢測(cè)，如果空閑時(shí)間大于timeBetweenEvictionRunsMillis，執(zhí)行validationQuery檢測(cè)連接是否有效。
validationQuery		用來(lái)檢測(cè)連接是否有效的sql，要求是一個(gè)查詢語(yǔ)句。如果validationQuery為null，testOnBorrow、testOnReturn、testWhileIdle都不會(huì)其作用。在mysql中通常為select 'x'，在oracle中通常為select 1 from dual
timeBetweenEviction-RunsMillis		1) Destroy線程會(huì)檢測(cè)連接的間隔時(shí)間 2) testWhileIdle的判斷依據(jù)
minEvictableIdle- TimeMillis		Destory線程中如果檢測(cè)到當(dāng)前連接的最后活躍時(shí)間和當(dāng)前時(shí)間的差值大于minEvictableIdleTimeMillis，則關(guān)閉當(dāng)前連接。
removeAbandoned		對(duì)于建立時(shí)間超過(guò)removeAbandonedTimeout的連接強(qiáng)制關(guān)閉
removeAbandoned-Timeout		指定連接建立多長(zhǎng)時(shí)間就需要被強(qiáng)制關(guān)閉
logAbandoned	false	指定發(fā)生removeabandoned的時(shí)候，是否記錄當(dāng)前線程的堆棧信息到日志中
filters		屬性類型是字符串，通過(guò)別名的方式配置擴(kuò)展插件，常用的插件有： 1）監(jiān)控統(tǒng)計(jì)用的filter:stat 2）日志用的filter:log4j 3）防御sql注入的filter:wall