1.缺陷一：Timer由于内部只要一个线程，管理多个任务的时候，一个任务延时，后面的任务就会跟着延时。用下面的例子说明：

package com.dxz.timer;import java.util.Timer;import java.util.TimerTask;public class TimerTaskTest {    private static long startTime;    public static void main(String[] args) {        // 创建一个定时task1        TimerTask task1 = new TimerTask() {            @Override            public void run() {                System.out.println("task1 excude ..." + (System.currentTimeMillis() - startTime));                try {                    Thread.sleep(3000);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        // 创建一个定时task2        TimerTask task2 = new TimerTask() {            @Override            public void run() {                System.out.println("task2 excude ..." + (System.currentTimeMillis() - startTime));                try {                    Thread.sleep(3000);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        Timer timer = new Timer("TimerThread");        startTime = System.currentTimeMillis();        // 第1个任务1s后，执行        timer.schedule(task1, 1000);        // 第2个任务3s后，执行        timer.schedule(task2, 3000);    }}

结果：

task1 excude ...1001task2 excude ...4002

从上面的代码看出，第一个任务1s后执行，第二个任务3s后执行。实际上task2是在4s后才执行的，因为Timer内部是一个线程，而task1所需的时间超过了两个任务的间隔时间导致。下面使用ScheduledThreadPool解决上面的问题：

package com.dxz.timer;import java.util.TimerTask;import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ScheduledThreadPoolTest1 {    private static long startTime;    public static void main(String[] args) {        ScheduledExecutorService newExecutorService = Executors.newScheduledThreadPool(2);        // 创建一个定时task1        TimerTask task1 = new TimerTask() {            @Override            public void run() {                System.out.println(Thread.currentThread().getName() + " excude ..."                        + (System.currentTimeMillis() - startTime) + "ms");                try {                    Thread.sleep(3000);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        // 创建一个定时task2        TimerTask task2 = new TimerTask() {            @Override            public void run() {                System.out.println(Thread.currentThread().getName() + " excude ..."                        + (System.currentTimeMillis() - startTime) + "ms");                try {                    Thread.sleep(3000);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        startTime = System.currentTimeMillis();        // 第1个任务1000 表示1s后，执行        newExecutorService.schedule(task1, 1000, TimeUnit.MILLISECONDS);        // 第2个任务3000 表示3s后，执行        newExecutorService.schedule(task2, 3000, TimeUnit.MILLISECONDS);    }}

结果：

pool-1-thread-1 excude ...1001mspool-1-thread-2 excude ...3002ms

2、缺陷二：

package com.dxz.timer;import java.util.Date;import java.util.Timer;import java.util.TimerTask;public class TimerTaskTest2 {    private static long startTime;    public static void main(String[] args) {        // 创建一个定时task1        TimerTask task1 = new TimerTask() {            @Override            public void run() {                throw new RuntimeException("运行时异常");            }        };        // 创建一个定时task2        TimerTask task2 = new TimerTask() {            @Override            public void run() {                System.out.println("task2 excude ..." + (System.currentTimeMillis() - startTime));                try {                    Thread.sleep(3000);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        Timer timer = new Timer("TimerThread");        startTime = System.currentTimeMillis();        timer.schedule(task1, 100);        timer.scheduleAtFixedRate(task2, new Date(), 1000);    }}

结果：

task2 excude ...2Exception in thread "TimerThread" java.lang.RuntimeException: 运行时异常    at com.dxz.timer.TimerTaskTest2$1.run(TimerTaskTest2.java:15)    at java.util.TimerThread.mainLoop(Timer.java:555)    at java.util.TimerThread.run(Timer.java:505)

使用ScheduledExecutorService来解决：

package com.dxz.timer;import java.util.TimerTask;import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ScheduledThreadPoolTest2 {    private static long startTime;    public static void main(String[] args) {        ScheduledExecutorService newExecutorService = Executors.newScheduledThreadPool(1);        // 创建一个定时task1        TimerTask task1 = new TimerTask() {            @Override            public void run() {                new RuntimeException();            }        };        // 创建一个定时task2        TimerTask task2 = new TimerTask() {            @Override            public void run() {                System.out.println(Thread.currentThread().getName() + " excude ..." + (System.currentTimeMillis() - startTime) + "ms");            }        };        startTime = System.currentTimeMillis();        newExecutorService.schedule(task1, 0, TimeUnit.MILLISECONDS);        // 第2个任务3000 表示3s后，执行        newExecutorService.scheduleAtFixedRate(task2,1000, 1000, TimeUnit.MILLISECONDS);    }}

结果：

pool-1-thread-1 excude ...1002mspool-1-thread-1 excude ...2001mspool-1-thread-1 excude ...3002mspool-1-thread-1 excude ...4002ms