图解java多线程-Immutable_模式

正文

使用 Immutable 模式的要点

实例创建后,状态不再发生变化

如下定义一个 Persion 类,注意 class 为 final,表示不能被继承,并且类的属性也是不可变的

public final class Persion {
    
    private final String name;
    private final String address;

    public Persion(String name, String address) {
        this.name = name;
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public String getAddress() {
        return address;
    }

    @Override
    public String toString() {
        return "Persion{" +
                "name='" + name + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

比如下面这个类就不是不可变的,原因在于 Point 不是不可变的,因为外部可能还保存 Point 的引用

public final class Line {

    private final Point startPoint;
    private final Point endPoint;

    public Line(Point startPoint, Point endPoint) {
        this.startPoint = startPoint;
        this.endPoint = endPoint;
    }

    public Point getStartPoint() {
        return startPoint;
    }

    public Point getEndPoint() {
        return endPoint;
    }

    @Override
    public String toString() {
        return "Line{" +
                "startPoint=" + startPoint +
                ", endPoint=" + endPoint +
                '}';
    }

    public static void main(String[] args) {
        Point startPoint = new Point(1,1);
        Point endPoint = new Point(2,2);
        Line line = new Line(startPoint,endPoint);
        System.out.println(line);
        startPoint.x = 3;
        startPoint.y = 3;
        System.out.println(line);
    }

    static final class Point{
        private int x;
        private int y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public String toString() {
            return "Point{" +
                    "x=" + x +
                    ", y=" + y +
                    '}';
        }
    }
}

运行结果:
Line{startPoint=Point{x=1, y=1}, endPoint=Point{x=2, y=2}}
Line{startPoint=Point{x=3, y=3}, endPoint=Point{x=2, y=2}}

上面的类可以将 Point 类改为不可变的,也可以改一下 Line 的构造方法,让外部不再持有引用,但是这样反射可能还是可以修改,一般这种类在程序中表示常量，而可以用枚举

public final class Line1 {

    private final Point startPoint;
    private final Point endPoint;

    public Line1(Point startPoint, Point endPoint) {
        this.startPoint = new Point(startPoint.x,startPoint.y);
        this.endPoint = new Point(endPoint.x,endPoint.y);
    }

    public Point getStartPoint() {
        return startPoint;
    }

    public Point getEndPoint() {
        return endPoint;
    }

    @Override
    public String toString() {
        return "Line1{" +
                "startPoint=" + startPoint +
                ", endPoint=" + endPoint +
                '}';
    }

    public static void main(String[] args) {
        Point startPoint = new Point(1,1);
        Point endPoint = new Point(2,2);
        Line1 line = new Line1(startPoint,endPoint);
        System.out.println(line);
        startPoint.x = 3;
        startPoint.y = 3;
        System.out.println(line);
        try {
            Field f = line.getClass().getDeclaredField("startPoint");
            Object obj = f.get(line);
            Field fx = obj.getClass().getDeclaredField("x");
            fx.setAccessible(true);
            Field fy = obj.getClass().getDeclaredField("y");
            fy.setAccessible(true);
            fx.set(obj,3);
            fy.set(obj,3);
            fx.setAccessible(false);
            fy.setAccessible(false);
            System.out.println(line);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    static final class Point{
        private int x;
        private int y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public String toString() {
            return "Point{" +
                    "x=" + x +
                    ", y=" + y +
                    '}';
        }
    }
}

运行结果:
Line1{startPoint=Point{x=1, y=1}, endPoint=Point{x=2, y=2}}
Line1{startPoint=Point{x=1, y=1}, endPoint=Point{x=2, y=2}}
Line1{startPoint=Point{x=3, y=3}, endPoint=Point{x=2, y=2}}

实例是共享的，且被频繁访问时

Immutable 模式的优点是"不需要使用synchronized进行保护".能够在不失去安全性和生存性的前提下提高性能

考虑成对的 mutable 类和 immutable 类

比如jdk中的 StringBuffer 和 String 类. StringBuffer 类表示字符串的 mutable 类.它表示字符串可以随便改写,为了确保安全,改写时需要妥善使用 synchronized.而 String 类表示字符串的 immutable 类
StringBuffer 类中有一个以 String 为参数的构造函数,而 String 类中也有一个以 StringBuffer 为参数的构造函数.也就是说, StringBuffer 和 String 实例可以互相转换

参考资料