除了常規(guī)的計(jì)算匹配文件數(shù)量外，這個(gè)程序打印出執(zhí)行過(guò)程中池中的最大線程數(shù)量。但從ExecutorService接口不能得到這個(gè)信息。因此，我們必須將pool對(duì)象轉(zhuǎn)型成一個(gè)ThreadPoolExecutor類(lèi)對(duì)象。
import java.io.*;
import java.util.*;
import java.util.concurrent.*; public class ThreadPoolTest
{
public static void main(String[] args) throws Exception
{
Scanner in = new Scanner(System.in);
System.out.print("Enter base directory (e.g. /usr/local/jdk5.0/src): ");
String directory = in.nextLine();
System.out.print("Enter keyword (e.g. volatile): ");
String keyword = in.nextLine(); ExecutorService pool = Executors.newCachedThreadPool(); MatchCounter counter = new MatchCounter(new File(directory), keyword, pool);
Future<Integer> result = pool.submit(counter); try
{
System.out.println(result.get() + " matching files.");
}
catch (ExecutionException e)
{
e.printStackTrace();
}
catch (InterruptedException e)
{
}
pool.shutdown(); int largestPoolSize = ((ThreadPoolExecutor) pool).getLargestPoolSize();
System.out.println("largest pool size=" + largestPoolSize);
}
} /**
* This task counts the files in a directory and its subdirectories that contain a given keyword.
*/
class MatchCounter implements Callable<Integer>
{
/**
* Constructs a MatchCounter.
* @param directory the directory in which to start the search
* @param keyword the keyword to look for
* @param pool the thread pool for submitting subtasks
*/
public MatchCounter(File directory, String keyword, ExecutorService pool)
{
this.directory = directory;
this.keyword = keyword;
this.pool = pool;
} public Integer call()
{
count = 0;
try
{
File[] files = directory.listFiles();
ArrayList<Future<Integer>> results = new ArrayList<Future<Integer>>(); for (File file : files)
if (file.isDirectory())
{
MatchCounter counter = new MatchCounter(file, keyword, pool);
Future<Integer> result = pool.submit(counter);
results.add(result);
}
else
{
if (search(file)) count++;
} for (Future<Integer> result : results)
try
{
count += result.get();
}
catch (ExecutionException e)
{
e.printStackTrace();
}
}
catch (InterruptedException e)
{
}
return count;
} /**
* Searches a file for a given keyword.
* @param file the file to search
* @return true if the keyword is contained in the file
*/
public boolean search(File file)
{
try
{
Scanner in = new Scanner(new FileInputStream(file));
boolean found = false;
while (!found && in.hasNextLine())
{
String line = in.nextLine();
if (line.contains(keyword)) found = true;
}
in.close();
return found;
}
catch (IOException e)
{
return false;
}
} private File directory;
private String keyword;
private ExecutorService pool;
private int count;
} |
構(gòu)建一個(gè)新的線程的代價(jià)還是有些高的，因?yàn)樗婕芭c
操作系統(tǒng)的交互。如果你的程序創(chuàng)建了大量生存期很短的線程，那就應(yīng)該使用線程池。一個(gè)線程池包含大量準(zhǔn)備運(yùn)行的空閑線程。你將一個(gè)Runnable對(duì)象給線程池，線程池中的一個(gè)線程就會(huì)調(diào)用run方法。當(dāng)run方法退出時(shí)，線程不會(huì)死亡，而是繼續(xù)在池中準(zhǔn)備為下一個(gè)請(qǐng)求提供服務(wù)。
執(zhí)行器（Executor）類(lèi)有大量用來(lái)構(gòu)建線程池的靜態(tài)工廠方法，下表給出了一個(gè)總結(jié)。
newCachedThreadPool、newFixedThreadPool和newSingleThreadExecutor這三個(gè)方法返回ThreadPoolExecutor類(lèi)（這個(gè)類(lèi)實(shí)現(xiàn)了ExecutorService接口）對(duì)象。
向線程池提交任務(wù)的方法為：將一個(gè)實(shí)現(xiàn)Runnable或Callable接口的對(duì)象提交給ExecutorService：
Future<?> submit(Runable task)
Future<T> submit(Runable task, T result)
Future<t> submit(Callable<T> task)
線程池會(huì)在適當(dāng)?shù)臅r(shí)候盡早執(zhí)行提交的任務(wù)，調(diào)用submit時(shí)會(huì)返回一個(gè)Future對(duì)象，用以查詢?cè)撊蝿?wù)的狀態(tài)，或者取消該任務(wù)。
第一個(gè)submit方法提交一個(gè)Runable對(duì)象返回一個(gè)Future<?>，可使用該對(duì)象調(diào)用isDone、cancel、或者isCancelled來(lái)查詢?nèi)蝿?wù)狀態(tài)。但是此Future對(duì)象的get方法在任務(wù)完成的時(shí)候知識(shí)簡(jiǎn)單的返回null
第二個(gè)版本的submit方法同樣提交一個(gè)Runable對(duì)象，并且返回Future的get方法在任務(wù)完成的時(shí)候返回傳入的result對(duì)象
第三個(gè)submit方法提交一個(gè)Callable對(duì)象，并且返回的Future對(duì)象將在計(jì)算結(jié)構(gòu)、準(zhǔn)備好的時(shí)候得到它。
當(dāng)想要注銷(xiāo)一個(gè)線程池，可調(diào)用shutdown方法，該方法啟動(dòng)該線程池的關(guān)閉序列。此時(shí)線程池并不是馬上就壯烈犧牲了線程也沒(méi)了，而是等待所以任務(wù)都完成以后，線程池中的線程才會(huì)死亡，被關(guān)閉的執(zhí)行器不再接受新任務(wù)。也可以調(diào)用shutdownNow，此時(shí)線程池會(huì)取消正在排隊(duì)等待處理的任務(wù)并且試圖中斷正在執(zhí)行的線程。
下面總結(jié)了在使用連接池時(shí)應(yīng)該做的事：
1、調(diào)用Executor類(lèi)中靜態(tài)的newCachedThreadPool或newFixedThreadPool方法。
2、調(diào)用submit來(lái)提交一個(gè)Runnable或Callable對(duì)象。
3、如果希望能夠取消任務(wù)或如果提交了一個(gè)Callable對(duì)象，那就保存好返回的Future對(duì)象。
4、當(dāng)不想再提交任何任務(wù)時(shí)調(diào)用shutdown。
除了常規(guī)的計(jì)算匹配文件數(shù)量外，這個(gè)程序打印出執(zhí)行過(guò)程中池中的最大線程數(shù)量。但從ExecutorService接口不能得到這個(gè)信息。因此，我們必須將pool對(duì)象轉(zhuǎn)型成一個(gè)ThreadPoolExecutor類(lèi)對(duì)象。
import java.io.*;
import java.util.*;
import java.util.concurrent.*; public class ThreadPoolTest
{
public static void main(String[] args) throws Exception
{
Scanner in = new Scanner(System.in);
System.out.print("Enter base directory (e.g. /usr/local/jdk5.0/src): ");
String directory = in.nextLine();
System.out.print("Enter keyword (e.g. volatile): ");
String keyword = in.nextLine(); ExecutorService pool = Executors.newCachedThreadPool(); MatchCounter counter = new MatchCounter(new File(directory), keyword, pool);
Future<Integer> result = pool.submit(counter); try
{
System.out.println(result.get() + " matching files.");
}
catch (ExecutionException e)
{
e.printStackTrace();
}
catch (InterruptedException e)
{
}
pool.shutdown(); int largestPoolSize = ((ThreadPoolExecutor) pool).getLargestPoolSize();
System.out.println("largest pool size=" + largestPoolSize);
}
} /**
* This task counts the files in a directory and its subdirectories that contain a given keyword.
*/
class MatchCounter implements Callable<Integer>
{
/**
* Constructs a MatchCounter.
* @param directory the directory in which to start the search
* @param keyword the keyword to look for
* @param pool the thread pool for submitting subtasks
*/
public MatchCounter(File directory, String keyword, ExecutorService pool)
{
this.directory = directory;
this.keyword = keyword;
this.pool = pool;
} public Integer call()
{
count = 0;
try
{
File[] files = directory.listFiles();
ArrayList<Future<Integer>> results = new ArrayList<Future<Integer>>(); for (File file : files)
if (file.isDirectory())
{
MatchCounter counter = new MatchCounter(file, keyword, pool);
Future<Integer> result = pool.submit(counter);
results.add(result);
}
else
{
if (search(file)) count++;
} for (Future<Integer> result : results)
try
{
count += result.get();
}
catch (ExecutionException e)
{
e.printStackTrace();
}
}
catch (InterruptedException e)
{
}
return count;
} /**
* Searches a file for a given keyword.
* @param file the file to search
* @return true if the keyword is contained in the file
*/
public boolean search(File file)
{
try
{
Scanner in = new Scanner(new FileInputStream(file));
boolean found = false;
while (!found && in.hasNextLine())
{
String line = in.nextLine();
if (line.contains(keyword)) found = true;
}
in.close();
return found;
}
catch (IOException e)
{
return false;
}
} private File directory;
private String keyword;
private ExecutorService pool;
private int count;
} |