Jakarta Commons項(xiàng)目研究--pool

2019-11-18 14:17:35

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一.pool項(xiàng)目
到底什么是 pool, 簡(jiǎn)單來說, 就是先建立一些存在的 object, 放在 pool 之中, 當(dāng)你有需要的時(shí)候,
可以從 pool 中直接獲取, 不需要重新建立.. 最常聽到的就是 database connection pooling,
因?yàn)榻?a href="http://www.49028c.com/sql.asp">數(shù)據(jù)庫連結(jié)是一件耗時(shí)的工作, 假如我們先把連結(jié)建立好, 就可以節(jié)省這一些時(shí)間。database
connection pooling 即DBCP。
1.基礎(chǔ)類結(jié)構(gòu)
pool的基礎(chǔ)類:
public interface ObjectPool {
Object borrowObject();
void returnObject(Object borrowed);
}
主要擴(kuò)展類:
public interface KeyedObjectPool {
Object borrowObject(Object key);
void returnObject(Object key, Object borrowed);
}按照key來索引pool的對(duì)象

基礎(chǔ)類僅提供了最基本的兩個(gè)函數(shù),用來創(chuàng)建和返回pool對(duì)象。
2.實(shí)現(xiàn)的基本類
BaSEObjectPool--PoolableObjectFactory--BasePoolableObjectFactory
KeyedObjectPool--KeyedPoolableObjectFactory--BaseKeyedPoolableObjectFactory
StackObjectPool--StackKeyedObjectPool--可以在初始化創(chuàng)建實(shí)例，提供有限的 idle 數(shù)量
GenericObjectPool--GenericKeyedObjectPool--包含了設(shè)定 idle, active
的數(shù)量以及回收到pool中的設(shè)置
SoftReferenceObjectPool--可以隨需要進(jìn)行增加,他的回收是由垃圾回收站進(jìn)行的
總的來說,它提供了 Pool 的 API 主要有三個(gè)方面:
提供一般性的對(duì)象 pool 接口, 可以簡(jiǎn)單地去使用和實(shí)現(xiàn). 比如BaseObjectPool和KeyedObjectPool.
提供小工具可以建立模塊化的 pool. 比如StackObjectPool.
實(shí)現(xiàn)出一些通用性的 pool. 比如GenericObjectPool.
3.實(shí)現(xiàn)

ObjectPool

Object obj = null;

try {
//創(chuàng)建對(duì)象
obj = pool.borrowObject();
//捕捉異常
} catch(Exception e) {
} finally {
//確認(rèn)對(duì)象是否已經(jīng)返回
if(null != obj) {
pool.returnObject(obj);
}
}

KeyedObjectPool

Object obj = null;
Object key = "Key";

try {
//創(chuàng)建對(duì)象
obj = pool.borrowObject(key);
//捕捉異常
} catch(Exception e) {
} finally {
//確認(rèn)返回
if(null != obj) {
pool.returnObject(key,obj);
}
}
其它的類型繼續(xù)開發(fā)就可以了
4.范例(取自apache的commons組)
ReaderUtil.java

import java.io.Reader;
import java.io.IOException;

public class ReaderUtil {

public ReaderUtil() {
}

/**
* Dumps the contents of the {@link Reader} to a
* String, closing the {@link Reader} when done.
*/
public String readToString(Reader in)
throws IOException {
StringBuffer buf = new StringBuffer();
try {

for( int c = in.read(); c != -1; c = in.read()) {
buf.append((char)c);
}
return buf.toString();
} catch(IOException e) {
throw e;
} finally {
try {
in.close();
} catch (Exception e) {
// ignored
}
}
}
}

改換一下順序，先取得pool，再由pool取值（推薦使用）

import org.apache.commons.pool.ObjectPool;
import java.io.Reader;
import java.io.IOException;

public class ReaderUtil {
PRivate ObjectPool pool;

public ReaderUtil(ObjectPool pool) {
this.pool = pool;
}

/**
* Dumps the contents of the {@link Reader} to a
* String, closing the {@link Reader} when done.
*/
public String readToString(Reader in) throws IOException {
StringBuffer buf = null;
try {
buf = (StringBuffer)(pool.borrowObject());
for(int c = in.read(); c != -1; c = in.read()) {
buf.append((char)c);
}
return buf.toString();
} catch(IOException e) {
throw e;
} catch(Exception e) {
throw new RuntimeException("Unable to borrow buffer from pool" +
e.toString());
} finally {
try {
in.close();
} catch(Exception e) {
// ignored
}
try {
if(null != buf) {
pool.returnObject(buf);
}
} catch(Exception e) {
// ignored
}
}
}
}

用StringBuffer做pool的例子(不推薦使用，僅供熟悉知識(shí))

StringBufferFactory.java

import org.apache.commons.pool.BasePoolableObjectFactory;

public class StringBufferFactory extends BasePoolableObjectFactory {
// for makeObject we′ll simply return a new buffer
public Object makeObject() {
return new StringBuffer();
}

// when an object is returned to the pool,
// we′ll clear it out
public void passivateObject(Object obj) {
StringBuffer buf = (StringBuffer)obj;
buf.setLength(0);
}

// for all other methods, the no-op
// implementation in BasePoolableObjectFactory
// will suffice
}

修改 ReaderUtil 由 StringBufferFactory Pool 得到 StringBuffer.

new ReaderUtil(
new StackObjectPool(
new StringBufferFactory()
)
)
5.總結(jié)
我們通常會(huì)在 io 的部分采用 pool 機(jī)制, 減少一些建立存取的時(shí)間, 對(duì)于最耗時(shí)的數(shù)據(jù)庫存取,
更是相對(duì)的重要,我將會(huì)在commons-DBCP專題進(jìn)行介紹，本篇是本系列的第二篇，以后將陸續(xù)推出，下期主題就是DBCP,請(qǐng)繼續(xù)關(guān)注.
6.參考 -- 相關(guān)書目或相關(guān)文章
*Jakarta Commons:
http://jakarta.apache.org/commons/
*Jakarta Commons Pool
http://jakarta.apache.org/commons/pool/
*Jakarta Commons Pool API:
http://jakarta.apache.org/commons/pool/apidocs/index.Html
*Oreilly: Using the Jakarta Commons, Part 3: #3
http://www.onjava.com/pub/a/onjava/2003/07/23/commons.html?page=3

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