計時器:
public static void main(String[] args) throws InterruptedException {
new Timer().schedule(new MyTask(), 3000); // 安排一個任務, 3秒之後執行
new Timer().schedule(new MyTask(), new Date(111, 10, 29, 10, 57, 45)); // 指定時間執行
new Timer().schedule(new MyTask(), 3000, 1000); // 3秒後第一次, 每隔1秒再來一次
new Timer().schedule(new MyTask(), new Date(111, 10, 29, 10, 59, 45),1000); // 指定時間執行第一次, 每隔1秒再來一次
while (true) {
System.out.println(new Date());
Thread.sleep(1000);
}
}
}
class MyTask extends TimerTask {
public void run() {
System.out.println("do something");
}
}
使用計時器安排任務,先2秒執行一次,然後4秒執行一次,再2秒執行一次,4秒執行一次,循環……:
public static void main(String[] args) throws InterruptedException {
new Timer().schedule(new Task1(), 2000);
while (true) {
System.out.println(new Date());
Thread.sleep(1000);
}
}
}
class Task1 extends TimerTask {
public void run() {
System.out.println("Task1");
new Timer().schedule(new Task2(), 4000);
}
}
class Task2 extends TimerTask {
public void run() {
System.out.println("Task2");
new Timer().schedule(new Task1(), 2000);
}
}
使用計時器安排任務,週一到週五每天凌晨4點執行:
public class Exercise2 {
public static void main(String[] args) {
Calendar c = Calendar.getInstance(); // 日曆, 指向當前時間
int hour = c.get(Calendar.HOUR_OF_DAY); // 當前小時
if(hour >= 4) // 如果過了4點, 就翻到明天
c.add(Calendar.DATE, 1);
c.set(Calendar.HOUR_OF_DAY, 4); // 設置爲4點整
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
new Timer().schedule(new Task(), c.getTime(), 1000 * 60 * 60 * 24);
}
}
class Task extends TimerTask {
public void run() {
int day = Calendar.getInstance().get(Calendar.DAY_OF_WEEK);
if(day == Calendar.SATURDAY || day == Calendar.SUNDAY)
return;
System.out.println("do something");
}
}
JDK1.4同步的三種方式:
同步代碼塊
使用synchronized(鎖對象){同步代碼}形式進行同步,多個線程執行同步代碼塊時如果使用的鎖對象相同,只能有一個線程執行。
同步方法
使用synchronized關鍵字修飾方法,這時整個方法都是同步的,使用this作爲鎖對象。
靜態同步方法
靜態方法也可以使用synchronized關鍵字修飾,方法內部的代碼也是同步的,這時的鎖對象是當前類的Class對象。
同步代碼塊:
public class SyncDemo1 {
public static void main(String[] args) {
final Printer p = new Printer();
new Thread(new Runnable() {
public void run() {
while (true)
p.print1();
}
}).start();
new Thread(new Runnable() {
public void run() {
while (true)
p.print2();
}
}).start();
}
}
class Printer {
private Object lock = new Object();
public void print1() {
synchronized (lock) {
System.out.print("清");
System.out.print("華");
System.out.print("大");
System.out.println("學");
}
}
public void print2() {
synchronized (lock) {
System.out.print("北");
System.out.print("京");
System.out.print("大");
System.out.println("學");
}
}
}
同步方法:
class Printer {
public synchronized void print1() {
System.out.print("清");
System.out.print("華");
System.out.print("大");
System.out.println("學");
}
public synchronized void print2() {
System.out.print("北");
System.out.print("京");
System.out.print("大");
System.out.println("學");
}
}
同步方法和同步代碼塊聯合使用實現同步(同步方法使用this當做鎖):
class Printer {
public void print1() {
synchronized (this) {
System.out.print("清");
System.out.print("華");
System.out.print("大");
System.out.println("學");
}
}
public synchronized void print2() {
System.out.print("北");
System.out.print("京");
System.out.print("大");
System.out.println("學");
}
}
同步方法和同步代碼塊聯合使用實現同步(靜態同步方法使用當前類的Class對象當做鎖):class Printer {
public void print1() {
synchronized (this.getClass()) {
System.out.print("清");
System.out.print("華");
System.out.print("大");
System.out.println("學");
}
}
public static synchronized void print2() {
System.out.print("北");
System.out.print("京");
System.out.print("大");
System.out.println("學");
}
}
循環嵌套同步鎖可能導致死鎖:
private static Object lock1 = new Object();
private static Object lock2 = new Object();
public static void main(String[] args) {
new Thread(new Runnable() {
public void run() {
synchronized (lock1) {
System.out.println("鎖定lock1");
synchronized (lock2) {
System.out.println("鎖定lock2");
}
System.out.println("釋放lock2");
}
System.out.println("釋放lock1");
}
}).start();
new Thread(new Runnable() {
public void run() {
synchronized (lock2) {
System.out.println("鎖定lock2");
synchronized (lock1) {
System.out.println("鎖定lock1");
}
System.out.println("釋放lock1");
}
System.out.println("釋放lock2");
}
}).start();
}