Java指令重排序在多线程环境下的处理方法

ublic class D {
    static Integer a;
    static Boolean flag;
    
    public static void writer() {
        a = 1;
        flag = true;
    }
    public static void reader() {
        if (flag != null && flag) {
            System.out.println(a);
            a = 0;
            flag = false;
        }
}

public static void writer() {
    a = 1;
    flag = true;
}

public class Singleton {
    private static UserModel instance;
    
    public static UserModel getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new UserModel(2, "B");
                }
            }
        }
        return instance;
    }
}

@Data
@AllArgsConstructor
class UserModel {
    private Integer userId;
    private String userName;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class ValueModel {
    private Integer value;
    private Boolean flag;
}

public class E {
    private static final AtomicReference<ValueModel> ar = new AtomicReference<>(new ValueModel());
    
    public static void writer() {
        ar.set(new ValueModel(1, true));
    }
    
    public static void reader() {
        ValueModel valueModel = ar.get();
        if (valueModel.getFlag() != null && valueModel.getFlag()) {
            System.out.println(valueModel.getValue());
            ar.set(new ValueModel(0, false));
        }
    }
}

public class Singleton {
    private volatile static UserModel instance;
    
    public static UserModel getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new UserModel(2, "B");
                }
            }
        }
        return instance;
    }
}

@Data
@AllArgsConstructor
class UserModel {
    private Integer userId;
    private String userName;