并发编程学习之四：线程池学习之一

灰太狼 2022-10-16 07:23 161阅读 0赞

什么是线程池，为什么使用线程池我这里就不多说了。

JDK中就提供了对线程池的支持。JDK提供了一套Executor框架。都是在java.util.concurrent包中，是JDK并发包的核心类。其中，ThreadPoolExecutor表示一个线程池。Executors就扮演着线程池工厂的角色。ThreadPoolExecutor类实现了Executors接口。因此通过这个接口，任何Runnable的对象都可以被ThreadPoolExecutor线程池调度。

Executor框架提供了各种类型的线程池，主要有以下工厂方法：

public static ExecutorService newFixedThreadPool(int nThreads);

public static ExecutorService new SingleThreadExecutor();

public static ExecutorService newCachedThreadPool();

public static ScheduledExecutorService newSingleThreadScheduledExecutor();

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSze);

以上这几种线程池的特点我们以后再说下面直接看一下这些代码：

public class ThreadPoolDemoFixed {
    
        public static class MyTask implements Runnable {
    
            @Override
            public void run() {
                System.out.println(System.currentTimeMillis() + ": Thread ID" + Thread.currentThread().getId());
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    
        public static void main(String[] args) {
            MyTask task = new MyTask();
            ExecutorService executorService = Executors.newFixedThreadPool(5);
            for (int i = 0; i < 10; i++) {
                executorService.submit(task);
            }
        }
    }

执行结果如下：

1622129185148: Thread ID10  
1622129185148: Thread ID12  
1622129185148: Thread ID11  
1622129185148: Thread ID13  
1622129185148: Thread ID14  
1622129190149: Thread ID13  
1622129190149: Thread ID12  
1622129190149: Thread ID14  
1622129190149: Thread ID10  
1622129190149: Thread ID11

前五行先一起打印出来，然后隔了五秒 后面五行一起被打印出来，说明

public static ExecutorService newFixedThreadPool(int nThreads);

创建了一个固定线程池大小的线程池，里面有五个线程，这五个线程执行完后别的任务才会被执行。

再来看下面这段代码：

public class ThreadPoolDemoCached {
    
        public static class MyTask implements Runnable {
    
            @Override
            public void run() {
                System.out.println(System.currentTimeMillis() + ": Thread ID" + Thread.currentThread().getId());
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    
        public static void main(String[] args) {
            MyTask task = new MyTask();
            ExecutorService executorService = Executors.newCachedThreadPool();
            for (int i = 0; i < 10; i++) {
                executorService.submit(task);
            }
        }
    }

结果如下：

1622129335702: Thread ID10  
1622129335703: Thread ID11  
1622129335703: Thread ID12  
1622129335703: Thread ID13  
1622129335704: Thread ID14  
1622129335705: Thread ID15  
1622129335706: Thread ID16  
1622129335706: Thread ID17  
1622129335706: Thread ID18  
1622129335706: Thread ID19

这十行是一下一起打印出来的，说明

public static ExecutorService newCachedThreadPool();

一下可以创建很多个线程去执行任务，为了做这个实验我把循环里面的 10 改为了100 ，就是说有100个任务需要执行，我们来看代码：

public class ThreadPoolDemoCached {
    
        public static class MyTask implements Runnable {
    
            @Override
            public void run() {
                System.out.println(System.currentTimeMillis() + ": Thread ID" + Thread.currentThread().getId());
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    
        public static void main(String[] args) {
            MyTask task = new MyTask();
            ExecutorService executorService = Executors.newCachedThreadPool();
            for (int i = 0; i < 100; i++) { // 这里改成了100
                executorService.submit(task);
            }
        }
    }

来看执行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=52128:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/tools.jar:/Users/jiangtao/IdeaProjects/demo/threaddemo/target/classes:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter-jdbc/2.4.5/spring-boot-starter-jdbc-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter/2.4.5/spring-boot-starter-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot/2.4.5/spring-boot-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-autoconfigure/2.4.5/spring-boot-autoconfigure-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter-logging/2.4.5/spring-boot-starter-logging-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/Users/jiangtao/java/tools/maven_repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/Users/jiangtao/java/tools/maven_repository/org/apache/logging/log4j/log4j-to-slf4j/2.13.3/log4j-to-slf4j-2.13.3.jar:/Users/jiangtao/java/tools/maven_repository/org/apache/logging/log4j/log4j-api/2.13.3/log4j-api-2.13.3.jar:/Users/jiangtao/java/tools/maven_repository/org/slf4j/jul-to-slf4j/1.7.30/jul-to-slf4j-1.7.30.jar:/Users/jiangtao/java/tools/maven_repository/jakarta/annotation/jakarta.annotation-api/1.3.5/jakarta.annotation-api-1.3.5.jar:/Users/jiangtao/java/tools/maven_repository/org/yaml/snakeyaml/1.27/snakeyaml-1.27.jar:/Users/jiangtao/java/tools/maven_repository/com/zaxxer/HikariCP/3.4.5/HikariCP-3.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/slf4j/slf4j-api/1.7.30/slf4j-api-1.7.30.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-jdbc/5.3.6/spring-jdbc-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-beans/5.3.6/spring-beans-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-tx/5.3.6/spring-tx-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter-web/2.4.5/spring-boot-starter-web-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter-json/2.4.5/spring-boot-starter-json-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/core/jackson-databind/2.11.4/jackson-databind-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/core/jackson-annotations/2.11.4/jackson-annotations-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/core/jackson-core/2.11.4/jackson-core-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/datatype/jackson-datatype-jdk8/2.11.4/jackson-datatype-jdk8-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/datatype/jackson-datatype-jsr310/2.11.4/jackson-datatype-jsr310-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.11.4/jackson-module-parameter-names-2.11.4.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/boot/spring-boot-starter-tomcat/2.4.5/spring-boot-starter-tomcat-2.4.5.jar:/Users/jiangtao/java/tools/maven_repository/org/apache/tomcat/embed/tomcat-embed-core/9.0.45/tomcat-embed-core-9.0.45.jar:/Users/jiangtao/java/tools/maven_repository/org/glassfish/jakarta.el/3.0.3/jakarta.el-3.0.3.jar:/Users/jiangtao/java/tools/maven_repository/org/apache/tomcat/embed/tomcat-embed-websocket/9.0.45/tomcat-embed-websocket-9.0.45.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-web/5.3.6/spring-web-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-webmvc/5.3.6/spring-webmvc-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-aop/5.3.6/spring-aop-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-context/5.3.6/spring-context-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-expression/5.3.6/spring-expression-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/mybatis/spring/boot/mybatis-spring-boot-starter/2.1.4/mybatis-spring-boot-starter-2.1.4.jar:/Users/jiangtao/java/tools/maven_repository/org/mybatis/spring/boot/mybatis-spring-boot-autoconfigure/2.1.4/mybatis-spring-boot-autoconfigure-2.1.4.jar:/Users/jiangtao/java/tools/maven_repository/org/mybatis/mybatis/3.5.6/mybatis-3.5.6.jar:/Users/jiangtao/java/tools/maven_repository/org/mybatis/mybatis-spring/2.0.6/mybatis-spring-2.0.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-core/5.3.6/spring-core-5.3.6.jar:/Users/jiangtao/java/tools/maven_repository/org/springframework/spring-jcl/5.3.6/spring-jcl-5.3.6.jar com.taojiang.threaddemo.pool.ThreadPoolDemoCached
    1622129426923: Thread ID10
    1622129426923: Thread ID11
    1622129426924: Thread ID12
    1622129426924: Thread ID13
    1622129426924: Thread ID14
    1622129426924: Thread ID15
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    1622129426942: Thread ID107
    1622129426942: Thread ID108
    1622129426942: Thread ID109
    
    Process finished with exit code 0

发现真的有100个任务一下就执行了，我们再加一句，等5秒后输出一句话，来看代码：

public class ThreadPoolDemoCachedTwo {
    
        public static class MyTask implements Runnable {
    
            @Override
            public void run() {
                System.out.println(System.currentTimeMillis() + ": Thread ID" + Thread.currentThread().getId());
                try {
                    Thread.sleep(5000);
                    System.out.println("等待了5秒了， Thread Id:" + Thread.currentThread().getId());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    
        public static void main(String[] args) {
            MyTask task = new MyTask();
            ExecutorService executorService = Executors.newCachedThreadPool();
            for (int i = 0; i < 10; i++) {
                executorService.submit(task);
            }
        }
    }

执行结果：

1622129582663: Thread ID10
    1622129582663: Thread ID12
    1622129582663: Thread ID11
    1622129582663: Thread ID13
    1622129582663: Thread ID14
    1622129582663: Thread ID15
    1622129582663: Thread ID16
    1622129582664: Thread ID17
    1622129582664: Thread ID18
    1622129582664: Thread ID19
    等待了5秒了， Thread Id:12
    等待了5秒了， Thread Id:14
    等待了5秒了， Thread Id:16
    等待了5秒了， Thread Id:18
    等待了5秒了， Thread Id:11
    等待了5秒了， Thread Id:13
    等待了5秒了， Thread Id:10
    等待了5秒了， Thread Id:19
    等待了5秒了， Thread Id:17
    等待了5秒了， Thread Id:15

前十行一起执行完，等了五秒，后面十行输出，所以

public static ExecutorService newCachedThreadPool();

newCachedThreadPool 缓存的线程池，其实是 corePoolSize 为 0， maxximumPoolSize 为无穷大的线程池，这意味着在没有任务时，该线程池内无线程，而当任务被提交时，该线程池便会立马创建线程去执行。但是newCachedThreadPool 又一个坏处，如果同时有大量的任务被提交，而任务的执行又不那么快时，那么系统便会开启等量的线程去处理，这样做可能会很快耗尽系统的资源。

今天先写到这里，明天继续学习。

以上的示例都来自于《java高并发程序设计（第二版）》，我是看了书照着书中的例子手敲了一下代码，加深一下印象，有兴趣的同学可以去看一下这本书，作者是葛一鸣，非常不错的书。