线程池之拒绝策略
当线程池中任务队列已满且最大线程数maximumPoolSize达到最大，此时如果还有任务过来，那么将会触发拒绝策略
线程池一共内置了四种拒绝策略,线程池默认使用AbortPolicy
private static final RejectedExecutionHandler defaultHandler =new AbortPolicy();
ThreadPoolExecutor.AbortPolicy()
丢弃任务并抛出RejectedExecutionException异常
public static class AbortPolicy implements RejectedExecutionHandler { public AbortPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { throw new RejectedExecutionException("Task " + r.toString() + " rejected from " + e.toString()); } }
ThreadPoolExecutor.DiscardPolicy()
丢弃任务，但是不抛出异常。
public static class DiscardPolicy implements RejectedExecutionHandler { public DiscardPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { } }
ThreadPoolExecutor.DiscardOldestPolicy()
丢弃队列最前面的任务，然后重新提交被拒绝的任务,e.getQueue().poll()将最前面的任务弹出
public static class DiscardOldestPolicy implements RejectedExecutionHandler { public DiscardOldestPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { e.getQueue().poll(); e.execute(r); } } }
ThreadPoolExecutor.CallerRunsPolicy()
由调用线程（提交任务的线程）处理该任务
public static class CallerRunsPolicy implements RejectedExecutionHandler { public CallerRunsPolicy() { } public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { r.run(); } } }
自定义拒绝策略
当上面拒绝策略不能满足我们的需求，我们需要自定义拒绝策略，只需要实现RejectedExecutionHandler就行
package thread.customthreadpool.reject; import java.util.concurrent.RejectedExecutionException; import java.util.concurrent.RejectedExecutionHandler; import java.util.concurrent.ThreadPoolExecutor; /** * 自定义拒绝策略 */ public class MyThreadRejectExecutionHandler implements RejectedExecutionHandler { @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { /** * 加入自己的处理逻辑 */ throw new RejectedExecutionException("请求异常"); } }
