Java concurrency线程池之线程池原理(四)_动力节点Java学院整理
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2023-12-15 08:27:58
拒绝策略介绍
线程池的拒绝策略，是指当任务添加到线程池中被拒绝，而采取的处理措施。
当任务添加到线程池中之所以被拒绝，可能是由于：第一，线程池异常关闭。第二，任...
拒绝策略介绍
线程池的拒绝策略，是指当任务添加到线程池中被拒绝，而采取的处理措施。
当任务添加到线程池中之所以被拒绝，可能是由于：第一，线程池异常关闭。第二，任务数量超过线程池的最大限制。
线程池共包括4种拒绝策略，它们分别是：abortpolicy, callerrunspolicy, discardoldestpolicy和discardpolicy。
- abortpolicy -- 当任务添加到线程池中被拒绝时，它将抛出 rejectedexecutionexception 异常。
- callerrunspolicy -- 当任务添加到线程池中被拒绝时，会在线程池当前正在运行的thread线程池中处理被拒绝的任务。
- discardoldestpolicy -- 当任务添加到线程池中被拒绝时，线程池会放弃等待队列中最旧的未处理任务，然后将被拒绝的任务添加到等待队列中。
- discardpolicy -- 当任务添加到线程池中被拒绝时，线程池将丢弃被拒绝的任务。
线程池默认的处理策略是abortpolicy！
拒绝策略对比和示例
下面通过示例，分别演示线程池的4种拒绝策略。
1. discardpolicy 示例
import java.lang.reflect.field;
import java.util.concurrent.arrayblockingqueue;
import java.util.concurrent.threadpoolexecutor;
import java.util.concurrent.timeunit;
import java.util.concurrent.threadpoolexecutor.discardpolicy;
public class discardpolicydemo {
private static final int threads_size = 1;
private static final int capacity = 1;
public static void main(string[] args) throws exception {
// 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size)，"线程池"的阻塞队列容量为1(capacity)。
threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds,
new arrayblockingqueue<runnable>(capacity));
// 设置线程池的拒绝策略为"丢弃"
pool.setrejectedexecutionhandler(new threadpoolexecutor.discardpolicy());
// 新建10个任务，并将它们添加到线程池中。
for (int i = 0; i < 10; i++) {
runnable myrun = new myrunnable("task-"+i);
pool.execute(myrun);
}
// 关闭线程池
pool.shutdown();
}
}
class myrunnable implements runnable {
private string name;
public myrunnable(string name) {
this.name = name;
}
@override
public void run() {
try {
system.out.println(this.name + " is running.");
thread.sleep(100);
} catch (exception e) {
e.printstacktrace();
}
}
}
运行结果：
task-0 is running. task-1 is running.
结果说明：线程池pool的"最大池大小"和"核心池大小"都为1(threads_size)，这意味着"线程池能同时运行的任务数量最大只能是1"。
线程池pool的阻塞队列是arrayblockingqueue，arrayblockingqueue是一个有界的阻塞队列，arrayblockingqueue的容量为1。这也意味着线程池的阻塞队列只能有一个线程池阻塞等待。
根据""中分析的execute()代码可知：线程池*运行了2个任务。第1个任务直接放到worker中，通过线程去执行；第2个任务放到阻塞队列中等待。其他的任务都被丢弃了！
2. discardoldestpolicy 示例
import java.lang.reflect.field;
import java.util.concurrent.arrayblockingqueue;
import java.util.concurrent.threadpoolexecutor;
import java.util.concurrent.timeunit;
import java.util.concurrent.threadpoolexecutor.discardoldestpolicy;
public class discardoldestpolicydemo {
private static final int threads_size = 1;
private static final int capacity = 1;
public static void main(string[] args) throws exception {
// 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size)，"线程池"的阻塞队列容量为1(capacity)。
threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds,
new arrayblockingqueue<runnable>(capacity));
// 设置线程池的拒绝策略为"discardoldestpolicy"
pool.setrejectedexecutionhandler(new threadpoolexecutor.discardoldestpolicy());
// 新建10个任务，并将它们添加到线程池中。
for (int i = 0; i < 10; i++) {
runnable myrun = new myrunnable("task-"+i);
pool.execute(myrun);
}
// 关闭线程池
pool.shutdown();
}
}
class myrunnable implements runnable {
private string name;
public myrunnable(string name) {
this.name = name;
}
@override
public void run() {
try {
system.out.println(this.name + " is running.");
thread.sleep(200);
} catch (exception e) {
e.printstacktrace();
}
}
}
运行结果：
task-0 is running. task-9 is running.
结果说明：将"线程池的拒绝策略"由discardpolicy修改为discardoldestpolicy之后，当有任务添加到线程池被拒绝时，线程池会丢弃阻塞队列中末尾的任务，然后将被拒绝的任务添加到末尾。
3. abortpolicy 示例
import java.lang.reflect.field;
import java.util.concurrent.arrayblockingqueue;
import java.util.concurrent.threadpoolexecutor;
import java.util.concurrent.timeunit;
import java.util.concurrent.threadpoolexecutor.abortpolicy;
import java.util.concurrent.rejectedexecutionexception;
public class abortpolicydemo {
private static final int threads_size = 1;
private static final int capacity = 1;
public static void main(string[] args) throws exception {
// 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size)，"线程池"的阻塞队列容量为1(capacity)。
threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds,
new arrayblockingqueue<runnable>(capacity));
// 设置线程池的拒绝策略为"抛出异常"
pool.setrejectedexecutionhandler(new threadpoolexecutor.abortpolicy());
try {
// 新建10个任务，并将它们添加到线程池中。
for (int i = 0; i < 10; i++) {
runnable myrun = new myrunnable("task-"+i);
pool.execute(myrun);
}
} catch (rejectedexecutionexception e) {
e.printstacktrace();
// 关闭线程池
pool.shutdown();
}
}
}
class myrunnable implements runnable {
private string name;
public myrunnable(string name) {
this.name = name;
}
@override
public void run() {
try {
system.out.println(this.name + " is running.");
thread.sleep(200);
} catch (exception e) {
e.printstacktrace();
}
}
}
(某一次)运行结果：
java.util.concurrent.rejectedexecutionexception at java.util.concurrent.threadpoolexecutor$abortpolicy.rejectedexecution(threadpoolexecutor.java:1774) at java.util.concurrent.threadpoolexecutor.reject(threadpoolexecutor.java:768) at java.util.concurrent.threadpoolexecutor.execute(threadpoolexecutor.java:656) at abortpolicydemo.main(abortpolicydemo.java:27) task-0 is running. task-1 is running.
结果说明：将"线程池的拒绝策略"由discardpolicy修改为abortpolicy之后，当有任务添加到线程池被拒绝时，会抛出rejectedexecutionexception。
4. callerrunspolicy 示例
import java.lang.reflect.field;
import java.util.concurrent.arrayblockingqueue;
import java.util.concurrent.threadpoolexecutor;
import java.util.concurrent.timeunit;
import java.util.concurrent.threadpoolexecutor.callerrunspolicy;
public class callerrunspolicydemo {
private static final int threads_size = 1;
private static final int capacity = 1;
public static void main(string[] args) throws exception {
// 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(threads_size)，"线程池"的阻塞队列容量为1(capacity)。
threadpoolexecutor pool = new threadpoolexecutor(threads_size, threads_size, 0, timeunit.seconds,
new arrayblockingqueue<runnable>(capacity));
// 设置线程池的拒绝策略为"callerrunspolicy"
pool.setrejectedexecutionhandler(new threadpoolexecutor.callerrunspolicy());
// 新建10个任务，并将它们添加到线程池中。
for (int i = 0; i < 10; i++) {
runnable myrun = new myrunnable("task-"+i);
pool.execute(myrun);
}
// 关闭线程池
pool.shutdown();
}
}
class myrunnable implements runnable {
private string name;
public myrunnable(string name) {
this.name = name;
}
@override
public void run() {
try {
system.out.println(this.name + " is running.");
thread.sleep(100);
} catch (exception e) {
e.printstacktrace();
}
}
}
(某一次)运行结果：
task-2 is running. task-3 is running. task-4 is running. task-5 is running. task-6 is running. task-7 is running. task-8 is running. task-9 is running. task-0 is running. task-1 is running.
结果说明：将"线程池的拒绝策略"由discardpolicy修改为callerrunspolicy之后，当有任务添加到线程池被拒绝时，线程池会将被拒绝的任务添加到"线程池正在运行的线程"中取运行