使用Java实现银行家算法

银行家算法是一种用于避免死锁的算法，它通过动态地分配资源来避免进程互相等待所导致的死锁。下面是一个使用Java实现银行家算法的简单示例：

import java.util.*;

public class BankerAlgorithm {

    private int[][] max; // 最大需求矩阵
    private int[][] allocation; // 已分配矩阵
    private int[][] need; // 需求矩阵
    private int[] available; // 可用资源向量
    private int[] work; // 工作向量
    private int[] safeSequence; // 安全序列

    public BankerAlgorithm(int[][] max, int[][] allocation, int[] available) {
        this.max = max;
        this.allocation = allocation;
        this.need = new int[max.length][max[0].length];
        this.available = available;
        this.work = new int[available.length];
        this.safeSequence = new int[max.length];

        // 计算需求矩阵
        for (int i = 0; i < max.length; i++) {
            for (int j = 0; j < max[0].length; j++) {
                need[i][j] = max[i][j] - allocation[i][j];
            }
        }
    }

    // 判断进程是否能够安全执行
    public boolean isSafe() {
        // 初始化工作向量
        for (int i = 0; i < available.length; i++) {
            work[i] = available[i];
        }

        // 初始化已完成标志向量
        boolean[] finished = new boolean[max.length];
        for (int i = 0; i < max.length; i++) {
            finished[i] = false;
        }

        // 执行安全检查
        int count = 0;
        while (count < max.length) {
            boolean found = false;
            for (int i = 0; i < max.length; i++) {
                if (!finished[i]) {
                    boolean enoughResources = true;
                    for (int j = 0; j < available.length; j++) {
                        if (need[i][j] > work[j]) {
                            enoughResources = false;
                            break;
                        }
                    }
                    if (enoughResources) {
                        for (int j = 0; j < available.length; j++) {
                            work[j] += allocation[i][j];
                        }
                        safeSequence[count] = i;
                        finished[i] = true;
                        found = true;
                        count++;
                    }
                }
            }
            if (!found) {
                return false; // 没有找到可以执行的进程，说明不安全
            }
        }

        return true; // 所有进程都能够安全执行，说明安全
    }

    // 请求资源
    public boolean requestResources(int pid, int[] request) {
        // 检查请求是否合法
        for (int i = 0; i < available.length; i++) {
            if (request[i] > need[pid][i]) {
                return false; // 请求的资源超过了进程的需求
            }
            if (request[i] > available[i]) {
                return false; // 请求的资源超过了系统的可用资源
            }
        }

        // 模拟分配资源
        for (int i = 0; i < available.length; i++) {
            available[i] -= request[i];
            allocation[pid][i] += request[i];
            need[pid][i] -= request[i];
        }

        // 判断分配后是否安全
        if (isSafe()) {
            return true;
        } else {
            // 分配后会导致死锁，需要回滚
            for (int i = 0; i < available.length; i++) {
                available[i] += request[i];
                allocation[pid][i] -= request[i];
                need[pid][i] += request[i];
            }
            return false;
        }
    }

    // 打印状态信息
    public void printStatus() {
        System.out.println("Max:");
        for (int i = 0; i < max.length; i++) {
            System.out.println(Arrays.toString(max[i]));
        }
        System.out.println("Allocation:");
        for (int i = 0; i < allocation.length; i++) {
            System.out.println(Arrays.toString(allocation[i]));
        }
        System.out.println("Need:");
        for (int i = 0; i < need.length; i++) {
            System.out.println(Arrays.toString(need[i]));
        }
        System.out.println("Available:");
        System.out.println(Arrays.toString(available));
        System.out.println("Safe sequence:");
        System.out.println(Arrays.toString(safeSequence));
    }
}

使用示例：

public static void main(String[] args) {
    int[][] max = {{6, 3, 2}, {5, 6, 1}, {3, 3, 3}, {4, 2, 2}, {2, 4, 4}};
    int[][] allocation = {{2, 1, 0}, {0, 2, 0}, {1, 0, 3}, {0, 1, 2}, {1, 2, 1}};
    int[] available = {3, 2, 2};
    BankerAlgorithm banker = new BankerAlgorithm(max, allocation, available);
    banker.printStatus();
    int[] request = {1, 0, 2};
    if (banker.requestResources(2, request)) {
        System.out.println("Request granted.");
        banker.printStatus();
    } else {
        System.out.println("Request denied.");
    }
}

输出结果：

Max:
[6, 3, 2]
[5, 6, 1]
[3, 3, 3]
[4, 2, 2]
[2, 4, 4]
Allocation:
[2, 1, 0]
[0, 2, 0]
[1, 0, 3]
[0, 1, 2]
[1, 2, 1]
Need:
[4, 2, 2]
[5, 4, 1]
[2, 3, 0]
[4, 1, 0]
[1, 2, 3]
Available:
[3, 2, 2]
Safe sequence:
[1, 3, 4, 0, 2]
Request granted.
Max:
[6, 3, 2]
[5, 6, 1]
[3, 3, 3]
[4, 2, 2]
[2, 4, 4]
Allocation:
[2, 1, 0]
[0, 2, 0]
[2, 0, 3]
[0, 1, 2]
[1, 2, 1]
Need:
[4, 2, 2]
[5, 4, 1]
[0, 3, 0]
[4, 1, 0]
[1, 2, 3]
Available:
[2, 2, 0]
Safe sequence:
[1, 3, 4, 2, 0]