線程池是多線程處理的一種形式，其中任務被添加到隊列中，然后在創(chuàng)建線程后由它們自己啟動，其實每個線程使用默認堆棧大小以默認優(yōu)先級執(zhí)行，武林技術頻道小編今天為大家?guī)碓斀饩€程池的原理與實現(xiàn)方法。
一. 線程池的簡介
通常我們使用多線程的方式是，需要時創(chuàng)建一個新的線程，在這個線程里執(zhí)行特定的任務，然后在任務完成后退出。這在一般的應用里已經(jīng)能夠滿足我們應用的需求，畢竟我們并不是什么時候都需要創(chuàng)建大量的線程，并在它們執(zhí)行一個簡單的任務后銷毀。
但是在一些web、email、database等應用里，比如彩鈴，我們的應用在任何時候都要準備應對數(shù)目巨大的連接請求，同時，這些請求所要完成的任務卻又可能非常的簡單，即只占用很少的處理時間。這時，我們的應用有可能處于不停的創(chuàng)建線程并銷毀線程的狀態(tài)。雖說比起進程的創(chuàng)建，線程的創(chuàng)建時間已經(jīng)大大縮短，但是如果需要頻繁的創(chuàng)建線程，并且每個線程所占用的處理時間又非常簡短，則線程創(chuàng)建和銷毀帶給處理器的額外負擔也是很可觀的。
線程池的作用正是在這種情況下有效的降低頻繁創(chuàng)建銷毀線程所帶來的額外開銷。一般來說，線程池都是采用預創(chuàng)建的技術，在應用啟動之初便預先創(chuàng)建一定數(shù)目的線程。應用在運行的過程中，需要時可以從這些線程所組成的線程池里申請分配一個空閑的線程，來執(zhí)行一定的任務，任務完成后，并不是將線程銷毀，而是將它返還給線程池，由線程池自行管理。如果線程池中預先分配的線程已經(jīng)全部分配完畢，但此時又有新的任務請求，則線程池會動態(tài)的創(chuàng)建新的線程去適應這個請求。當然，有可能，某些時段應用并不需要執(zhí)行很多的任務，導致了線程池中的線程大多處于空閑的狀態(tài)，為了節(jié)省系統(tǒng)資源，線程池就需要動態(tài)的銷毀其中的一部分空閑線程。因此，線程池都需要一個管理者，按照一定的要求去動態(tài)的維護其中線程的數(shù)目。
基于上面的技術，線程池將頻繁創(chuàng)建和銷毀線程所帶來的開銷分攤到了每個具體執(zhí)行的任務上，執(zhí)行的次數(shù)越多，則分攤到每個任務上的開銷就越小。
當然，如果線程創(chuàng)建銷毀所帶來的開銷與線程執(zhí)行任務的開銷相比微不足道，可以忽略不計，則線程池并沒有使用的必要。比如，F(xiàn)TP、Telnet等應用時。
二. 線程池的設計
下面利用C語言來實現(xiàn)一個簡單的線程池，為了使得這個線程池庫使用起來更加方便，特在C實現(xiàn)中加入了一些OO的思想，與Objective-C不同，它僅僅是使用了struct來模擬了c++中的類，其實這種方式在linux內核中大量可見。
在這個庫里，與用戶有關的接口主要有:
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?????? typedef struct tp_work_desc_s tp_work_desc; //應用線程執(zhí)行任務時所需要的一些信息
?????? typedef struct tp_work_s tp_work; //線程執(zhí)行的任務
?????? typedef struct tp_thread_info_s tp_thread_info; //描述了各個線程id，是否空閑，執(zhí)行的任務等信息
?????? typedef struct tp_thread_pool_s tp_thread_pool; // 有關線程池操作的接口信息
???????? //thread parm
?????? struct tp_work_desc_s{
????????????????? ……
??????? };
?????? //base thread struct
?????? struct tp_work_s{
????????????????? //main process function. user interface
????????????????? void (*process_job)(tp_work *this, tp_work_desc *job);
??????? };
??????? tp_thread_pool *creat_thread_pool(int min_num, int max_num);
tp_work_desc_s表示應用線程執(zhí)行任務時所需要的一些信息，會被當作線程的參數(shù)傳遞給每個線程，依據(jù)應用的不同而不同，需要用戶定義結構的內容。tp_work_s就是我們希望線程執(zhí)行的任務了。當我們申請分配一個新的線程時，首先要明確的指定這兩個結構，即該線程完成什么任務，并且完成這個任務需要哪些額外的信息。接口函數(shù)creat_thread_pool用來創(chuàng)建一個線程池的實例，使用時需要指定該線程池實例所能容納的最小線程數(shù)min_num和最大線程數(shù)max_num。最小線程數(shù)即線程池創(chuàng)建時預創(chuàng)建的線程數(shù)目，這個數(shù)目的大小也直接影響了線程池所能起到的效果，如果指定的太小，線程池中預創(chuàng)建的線程很快就將分配完畢并需要創(chuàng)建新的線程來適應不斷的請求，如果指定的太大，則將可能會有大量的空閑線程。我們需要根據(jù)自己應用的實際需要進行指定。描述線程池的結構如下：
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??????? //main thread pool struct
??????? struct tp_thread_pool_s{
???????????? TPBOOL (*init)(tp_thread_pool *this);
???????????? void (*close)(tp_thread_pool *this);
???????????? void (*process_job)(tp_thread_pool *this, tp_work *worker, tp_work_desc *job);
???????????? int? (*get_thread_by_id)(tp_thread_pool *this, int id);
???????????? TPBOOL (*add_thread)(tp_thread_pool *this);
???????????? TPBOOL (*delete_thread)(tp_thread_pool *this);
????????????? int (*get_tp_status)(tp_thread_pool *this);
????????????? int min_th_num;??????????????? //min thread number in the pool
????????????? int cur_th_num;???????????????? //current thread number in the pool
????????????? int max_th_num;???????? //max thread number in the pool
????????????? pthread_mutex_t tp_lock;
????????????? pthread_t manage_thread_id;? //manage thread id num
????????????? tp_thread_info *thread_info;?? //work thread relative thread info
};
???????? 結構tp_thread_info_s描述了各個線程id、是否空閑、執(zhí)行的任務等信息，用戶并不需要關心它。
???????? //thread info
???????? struct tp_thread_info_s{
????????????? pthread_t????????? thread_id;???????? //thread id num
???????????? TPBOOL?????????????????? is_busy;??? //thread status:true-busy;flase-idle
???????????? pthread_cond_t????????? thread_cond;
???????????? pthread_mutex_t?????????????? thread_lock;
???????????? tp_work????????????????????? *th_work;
???????????? tp_work_desc??????????? *th_job;
???????? };
tp_thread_pool_s結構包含了有關線程池操作的接口和變量。在使用creat_thread_pool返回一個線程池實例之后，首先要使用明確使用init接口對它進行初始化。在這個初始化過程中，線程池會預創(chuàng)建指定的最小線程數(shù)目的線程，它們都處于阻塞狀態(tài)，并不損耗CPU，但是會占用一定的內存空間。同時init也會創(chuàng)建一個線程池的管理線程，這個線程會在線程池的運行周期內一直執(zhí)行，它將定時的查看分析線程池的狀態(tài)，如果線程池中空閑的線程過多，它會刪除部分空閑的線程，當然它并不會使所有線程的數(shù)目小于指定的最小線程數(shù)。
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在已經(jīng)創(chuàng)建并初始化了線程池之后，我們就可以指定tp_work_desc_s和tp_work_s結構，并使用線程池的process_job接口來執(zhí)行它們。這些就是我們使用這個線程池時所需要了解的所有東西。如果不再需要線程池，可以使用close接口銷毀它。
三. 實現(xiàn)代碼
Thread-pool.h(頭文件)：
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#include <stdio.h>??
#include <stdlib.h>??
#include <sys/types.h>??
#include <pthread.h>??
#include <signal.h>??
#ifndef TPBOOL??
typedef int TPBOOL;?
#endif??
#ifndef TRUE??
#define TRUE 1??
#endif??
#ifndef FALSE??
#define FALSE 0??
#endif??
#define BUSY_THRESHOLD 0.5? //(busy thread)/(all thread threshold)??
#define MANAGE_INTERVAL 5?? //tp manage thread sleep interval??
typedef struct tp_work_desc_s tp_work_desc;?
typedef struct tp_work_s tp_work;?
typedef struct tp_thread_info_s tp_thread_info;?
typedef struct tp_thread_pool_s tp_thread_pool;?
//thread parm??
struct tp_work_desc_s{?
??? char *inum; //call in??
??? char *onum; //call out??
??? int chnum;? //channel num??
};?
//base thread struct??
struct tp_work_s{?
??? //main process function. user interface??
??? void (*process_job)(tp_work *this, tp_work_desc *job);?
};?
//thread info??
struct tp_thread_info_s{?
??? pthread_t?????? thread_id;? //thread id num??
??? TPBOOL????????? is_busy;??? //thread status:true-busy;flase-idle??
??? pthread_cond_t????????? thread_cond;?????
??? pthread_mutex_t???? thread_lock;?
??? tp_work???????? *th_work;?
??? tp_work_desc??????? *th_job;?
};?
//main thread pool struct??
struct tp_thread_pool_s{?
??? TPBOOL (*init)(tp_thread_pool *this);?
??? void (*close)(tp_thread_pool *this);?
??? void (*process_job)(tp_thread_pool *this, tp_work *worker, tp_work_desc *job);?
??? int? (*get_thread_by_id)(tp_thread_pool *this, int id);?
??? TPBOOL (*add_thread)(tp_thread_pool *this);?
??? TPBOOL (*delete_thread)(tp_thread_pool *this);?
??? int (*get_tp_status)(tp_thread_pool *this);?
??? int min_th_num;???? //min thread number in the pool??
??? int cur_th_num;???? //current thread number in the pool??
??? int max_th_num;???????? //max thread number in the pool??
??? pthread_mutex_t tp_lock;?
??? pthread_t manage_thread_id; //manage thread id num??
??? tp_thread_info *thread_info;??? //work thread relative thread info??
};?
tp_thread_pool *creat_thread_pool(int min_num, int max_num);
Thread-pool.c(實現(xiàn)文件)：
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#include "thread-pool.h"??
static void *tp_work_thread(void *pthread);?
static void *tp_manage_thread(void *pthread);?
static TPBOOL tp_init(tp_thread_pool *this);?
static void tp_close(tp_thread_pool *this);?
static void tp_process_job(tp_thread_pool *this, tp_work *worker, tp_work_desc *job);?
static int? tp_get_thread_by_id(tp_thread_pool *this, int id);?
static TPBOOL tp_add_thread(tp_thread_pool *this);?
static TPBOOL tp_delete_thread(tp_thread_pool *this);?
static int? tp_get_tp_status(tp_thread_pool *this);?
/**
? * user interface. creat thread pool.
? * para:
? *???? num: min thread number to be created in the pool
? * return:
? *???? thread pool struct instance be created successfully
? */?
tp_thread_pool *creat_thread_pool(int min_num, int max_num){?
??? tp_thread_pool *this;?
??? this = (tp_thread_pool*)malloc(sizeof(tp_thread_pool));??
??? memset(this, 0, sizeof(tp_thread_pool));?
??? //init member function ponter??
??? this->init = tp_init;?
??? this->close = tp_close;?
??? this->process_job = tp_process_job;?
??? this->get_thread_by_id = tp_get_thread_by_id;?
??? this->add_thread = tp_add_thread;?
??? this->delete_thread = tp_delete_thread;?
??? this->get_tp_status = tp_get_tp_status;?
??? //init member var??
??? this->min_th_num = min_num;?
??? this->cur_th_num = this->min_th_num;?
??? this->max_th_num = max_num;?
??? pthread_mutex_init(&this->tp_lock, NULL);?
??? //malloc mem for num thread info struct??
??? if(NULL != this->thread_info)?
??????? free(this->thread_info);?
??? this->thread_info = (tp_thread_info*)malloc(sizeof(tp_thread_info)*this->max_th_num);?
??? return this;?
}?
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/**
? * member function reality. thread pool init function.
? * para:
? *???? this: thread pool struct instance ponter
? * return:
? *???? true: successful; false: failed
? */?
TPBOOL tp_init(tp_thread_pool *this){?
??? int i;?
??? int err;?
??? //creat work thread and init work thread info??
??? for(i=0;i<this->min_th_num;i++){?
??????? pthread_cond_init(&this->thread_info[i].thread_cond, NULL);?
??????? pthread_mutex_init(&this->thread_info[i].thread_lock, NULL);?
??????? err = pthread_create(&this->thread_info[i].thread_id, NULL, tp_work_thread, this);?
??????? if(0 != err){?
??????????? printf("tp_init: creat work thread failed/n");?
??????????? return FALSE;?
??????? }?
??????? printf("tp_init: creat work thread %d/n", this->thread_info[i].thread_id);?
??? }?
??? //creat manage thread??
??? err = pthread_create(&this->manage_thread_id, NULL, tp_manage_thread, this);?
??? if(0 != err){?
??????? printf("tp_init: creat manage thread failed/n");?
??????? return FALSE;?
??? }?
??? printf("tp_init: creat manage thread %d/n", this->manage_thread_id);?
??? return TRUE;?
}?
/**
? * member function reality. thread pool entirely close function.
? * para:
? *???? this: thread pool struct instance ponter
? * return:
? */?
void tp_close(tp_thread_pool *this){?
??? int i;?
??? //close work thread??
??? for(i=0;i<this->cur_th_num;i++){?
??????? kill(this->thread_info[i].thread_id, SIGKILL);?
??????? pthread_mutex_destroy(&this->thread_info[i].thread_lock);?
??????? pthread_cond_destroy(&this->thread_info[i].thread_cond);?
??????? printf("tp_close: kill work thread %d/n", this->thread_info[i].thread_id);?
??? }?
??? //close manage thread??
??? kill(this->manage_thread_id, SIGKILL);?
??? pthread_mutex_destroy(&this->tp_lock);?
??? printf("tp_close: kill manage thread %d/n", this->manage_thread_id);?
??? //free thread struct??
??? free(this->thread_info);?
}?
/**
? * member function reality. main interface opened.?
? * after getting own worker and job, user may use the function to process the task.
? * para:
? *???? this: thread pool struct instance ponter
? * worker: user task reality.
? * job: user task para
? * return:
? */?
void tp_process_job(tp_thread_pool *this, tp_work *worker, tp_work_desc *job){?
??? int i;?
??? int tmpid;?
??? //fill this->thread_info's relative work key??
??? for(i=0;i<this->cur_th_num;i++){?
??????? pthread_mutex_lock(&this->thread_info[i].thread_lock);?
??????? if(!this->thread_info[i].is_busy){?
??????????? printf("tp_process_job: %d thread idle, thread id is %d/n", i, this->thread_info[i].thread_id);?
??????????? //thread state be set busy before work??
??????????? this->thread_info[i].is_busy = TRUE;?
??????????? pthread_mutex_unlock(&this->thread_info[i].thread_lock);?
??????????? this->thread_info[i].th_work = worker;?
??????????? this->thread_info[i].th_job = job;?
??????????? printf("tp_process_job: informing idle working thread %d, thread id is %d/n", i, this->thread_info[i].thread_id);?
??????????? pthread_cond_signal(&this->thread_info[i].thread_cond);?
??????????? return;?
??????? }?
??????? else??
??????????? pthread_mutex_unlock(&this->thread_info[i].thread_lock);??????
??? }//end of for??
??? //if all current thread are busy, new thread is created here??
??? pthread_mutex_lock(&this->tp_lock);?
??? if( this->add_thread(this) ){?
??????? i = this->cur_th_num - 1;?
??????? tmpid = this->thread_info[i].thread_id;?
??????? this->thread_info[i].th_work = worker;?
??????? this->thread_info[i].th_job = job;?
??? }?
??? pthread_mutex_unlock(&this->tp_lock);?
??? //send cond to work thread??
??? printf("tp_process_job: informing idle working thread %d, thread id is %d/n", i, this->thread_info[i].thread_id);?
??? pthread_cond_signal(&this->thread_info[i].thread_cond);?
??? return;??
}?
/**
? * member function reality. get real thread by thread id num.
? * para:
? *???? this: thread pool struct instance ponter
? * id: thread id num
? * return:
? *???? seq num in thread info struct array
? */?
int tp_get_thread_by_id(tp_thread_pool *this, int id){?
??? int i;?
??? for(i=0;i<this->cur_th_num;i++){?
??????? if(id == this->thread_info[i].thread_id)?
??????????? return i;?
??? }?
??? return -1;?
}?
/**
? * member function reality. add new thread into the pool.
? * para:
? *???? this: thread pool struct instance ponter
? * return:
? *???? true: successful; false: failed
? */?
static TPBOOL tp_add_thread(tp_thread_pool *this){?
??? int err;?
??? tp_thread_info *new_thread;?
??? if( this->max_th_num <= this->cur_th_num )?
??????? return FALSE;?
??? //malloc new thread info struct??
??? new_thread = &this->thread_info[this->cur_th_num];?
??? //init new thread's cond & mutex??
??? pthread_cond_init(&new_thread->thread_cond, NULL);?
??? pthread_mutex_init(&new_thread->thread_lock, NULL);?
??? //init status is busy??
??? new_thread->is_busy = TRUE;?
??? //add current thread number in the pool.??
??? this->cur_th_num++;?
??? err = pthread_create(&new_thread->thread_id, NULL, tp_work_thread, this);?
??? if(0 != err){?
??????? free(new_thread);?
??????? return FALSE;?
??? }?
??? printf("tp_add_thread: creat work thread %d/n", this->thread_info[this->cur_th_num-1].thread_id);?
??? return TRUE;?
}?
/**
? * member function reality. delete idle thread in the pool.
? * only delete last idle thread in the pool.
? * para:
? *???? this: thread pool struct instance ponter
? * return:
? *???? true: successful; false: failed
? */?
static TPBOOL tp_delete_thread(tp_thread_pool *this){?
??? //current thread num can't < min thread num??
??? if(this->cur_th_num <= this->min_th_num) return FALSE;?
??? //if last thread is busy, do nothing??
??? if(this->thread_info[this->cur_th_num-1].is_busy) return FALSE;?
??? //kill the idle thread and free info struct??
??? kill(this->thread_info[this->cur_th_num-1].thread_id, SIGKILL);?
??? pthread_mutex_destroy(&this->thread_info[this->cur_th_num-1].thread_lock);?
??? pthread_cond_destroy(&this->thread_info[this->cur_th_num-1].thread_cond);?
??? //after deleting idle thread, current thread num -1??
??? this->cur_th_num--;?
??? return TRUE;?
}?
/**
? * member function reality. get current thread pool status:idle, normal, busy, .etc.
? * para:
? *???? this: thread pool struct instance ponter
? * return:
? *???? 0: idle; 1: normal or busy(don't process)
? */?
static int? tp_get_tp_status(tp_thread_pool *this){?
??? float busy_num = 0.0;?
??? int i;?
??? //get busy thread number??
??? for(i=0;i<this->cur_th_num;i++){?
??????? if(this->thread_info[i].is_busy)?
??????????? busy_num++;?
??? }?
??? //0.2? or other num???
??? if(busy_num/(this->cur_th_num) < BUSY_THRESHOLD)?
??????? return 0;//idle status??
??? else?
??????? return 1;//busy or normal status??????
}?
/**
? * internal interface. real work thread.
? * para:
? *???? pthread: thread pool struct ponter
? * return:
? */?
static void *tp_work_thread(void *pthread){?
??? pthread_t curid;//current thread id??
??? int nseq;//current thread seq in the this->thread_info array??
??? tp_thread_pool *this = (tp_thread_pool*)pthread;//main thread pool struct instance??
??? //get current thread id??
??? curid = pthread_self();?
??? //get current thread's seq in the thread info struct array.??
??? nseq = this->get_thread_by_id(this, curid);?
??? if(nseq < 0)?
??????? return;?
??? printf("entering working thread %d, thread id is %d/n", nseq, curid);?
??? //wait cond for processing real job.??
??? while( TRUE ){?
??????? pthread_mutex_lock(&this->thread_info[nseq].thread_lock);?
??????? pthread_cond_wait(&this->thread_info[nseq].thread_cond, &this->thread_info[nseq].thread_lock);?
??????? pthread_mutex_unlock(&this->thread_info[nseq].thread_lock);???????
??????? printf("%d thread do work!/n", pthread_self());?
??????? tp_work *work = this->thread_info[nseq].th_work;?
??????? tp_work_desc *job = this->thread_info[nseq].th_job;?
??????? //process??
??????? work->process_job(work, job);?
??????? //thread state be set idle after work??
??????? pthread_mutex_lock(&this->thread_info[nseq].thread_lock);?????????
??????? this->thread_info[nseq].is_busy = FALSE;?
??????? pthread_mutex_unlock(&this->thread_info[nseq].thread_lock);?
??????? printf("%d do work over/n", pthread_self());?
??? }????
}?
/**
? * internal interface. manage thread pool to delete idle thread.
? * para:
? *???? pthread: thread pool struct ponter
? * return:
? */?
static void *tp_manage_thread(void *pthread){?
??? tp_thread_pool *this = (tp_thread_pool*)pthread;//main thread pool struct instance??
??? //1???
??? sleep(MANAGE_INTERVAL);?
??? do{?
??????? if( this->get_tp_status(this) == 0 ){?
??????????? do{?
??????????????? if( !this->delete_thread(this) )?
??????????????????? break;?
??????????? }while(TRUE);?
??????? }//end for if??
??????? //1???
??????? sleep(MANAGE_INTERVAL);?
??? }while(TRUE);?
}?
四. 數(shù)據(jù)庫連接池介紹???
數(shù)據(jù)庫連接是一種關鍵的有限的昂貴的資源，這一點在多用戶的網(wǎng)頁應用程序中體現(xiàn)得尤為突出。
一個數(shù)據(jù)庫連接對象均對應一個物理數(shù)據(jù)庫連接，每次操作都打開一個物理連接，使用完都關閉連接，這樣造成系統(tǒng)的 性能低下。 數(shù)據(jù)庫連接池的解決方案是在應用程序啟動時建立足夠的數(shù)據(jù)庫連接，并講這些連接組成一個連接池(簡單說：在一個“池”里放了好多半成品的數(shù)據(jù)庫聯(lián)接對象)，由應用程序動態(tài)地對池中的連接進行申請、使用和釋放。對于多于連接池中連接數(shù)的并發(fā)請求，應該在請求隊列中排隊等待。并且應用程序可以根據(jù)池中連接的使用率，動態(tài)增加或減少池中的連接數(shù)。
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連接池技術盡可能多地重用了消耗內存地資源，大大節(jié)省了內存，提高了服務器地服務效率，能夠支持更多的客戶服務。通過使用連接池，將大大提高程序運行效率，同時，我們可以通過其自身的管理機制來監(jiān)視數(shù)據(jù)庫連接的數(shù)量、使用情況等。
1)? 最小連接數(shù)是連接池一直保持的數(shù)據(jù)庫連接，所以如果應用程序對數(shù)據(jù)庫連接的使用量不大，將會有大量的數(shù)據(jù)庫連接資源被浪費；
2)? 最大連接數(shù)是連接池能申請的最大連接數(shù)，如果數(shù)據(jù)庫連接請求超過此數(shù)，后面的數(shù)據(jù)庫連接請求將被加入到等待隊列中，這會影響之后的數(shù)據(jù)庫操作。
其實每個行業(yè)都有發(fā)光發(fā)熱的人，我們要與時俱進，在該學習的時候要認真學習，不要讓我們學習到的專業(yè)知識成為垃圾，當然我們也不要氣餒，武林技術頻道會陪著大家。
