0 同步機制遵循準則
- 空閑讓進:臨界區空閑時,允許請求進入臨界區
- 忙則等待:有線程進入臨界區時,其他線程必須等待
- 有限等待:等待不能無限制持續下去
- 讓權等待:釋放等待線程的 CPU 資源
1 單标志
問題:違背空閑讓進(當有一個線程結束,另外一個即使沒有結束也無法進入臨界區)
public class SingleFlag {
public static volatile boolean flag;
public static volatile int num = 0;
public static void main(String[] args) {
new Thread(() -> {
while (true) {
while (flag) ;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag = false;
}
}).start();
new Thread(() -> {
while (true) {
while (flag) ;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag = true;
}
}).start();
}
}
2 雙标志先檢查
問題:違背忙則等待(恰好執行完 1 時線程切換,此時 2 判斷也能通過,則會存在安全問題)
public class DoubleFlag {
public static volatile boolean[] flag={false, true};
public static volatile int num = 0;
public static void main(String[] args) {
new Thread(() -> {
while (true) {
while (flag[0]); // 1
flag[1] = true;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[1] = false;
}
}).start();
new Thread(() -> {
while (true) {
while (flag[1]); // 2
flag[0] = true;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[0] = false;
}
}).start();
}
}
3 雙标志後檢查
問題:違背有限等待原則(兩個線程無限制謙讓,導緻活鎖)
public class DoubleFlagAfterCheck {
public static volatile boolean[] flag = {false, true};
public static volatile int num = 0;
public static void main(String[] args) {
new Thread(() -> {
while (true) {
flag[1] = true;
while (flag[0]);
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[1] = false;
}
}).start();
new Thread(() -> {
while (true) {
flag[0] = true;
while (flag[1]);
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[0] = false;
}
}).start();
}
}
4 Peterson
完美解決并發安全性問題
問題:無法達到讓權等待(CPU 一直在死循環判斷,占用 CPU 資源)
public class Peterson {
public static volatile boolean[] flag = {false, false};
public static volatile int turn = 0;
public static volatile int num = 0;
public static void main(String[] args) {
new Thread(() -> {
while (true) {
flag[0] = true;
turn = 2;
while (flag[1] && turn == 2) ;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[0] = false;
}
}).start();
new Thread(() -> {
while (true) {
flag[1] = true;
turn = 1;
while (flag[0] && turn == 1) ;
System.out.println(Thread.currentThread().getName() + " " + ++num);
flag[1] = false;
}
}).start();
}
}