Hashset 使用不当引起的内存泄漏

傷城~ 2022-07-12 01:28 529阅读 0赞

修改hashset中对象的属性值，且属性值是计算哈希值的字段，这时会引起内存泄漏

即：当一个对象被存储进HashSet集合中以后，就不能修改该对象的参与计算哈希值的属性值了  
，否则对象修改后的哈希值与最初存储进HashSet集合中时的哈希值就不同了，在这种情况下，即使在contains方法使用该对象的当前引用作为参数去HashSet集合中检索对象，也将返回找不到对象的结果，这也会导致无法从HashSet集合中删除当前对象，造成内存泄露。

举例说明：

public class HashSetTest {
        public static void main(String[] args) {
            Set<Person> set = new HashSet<Person>();
            Person p1 = new Person("唐僧", "pwd1", 25);
            Person p2 = new Person("孙悟空", "pwd2", 26);
            Person p3 = new Person("猪八戒", "pwd3", 27);
            set.add(p1);
            set.add(p2);
            set.add(p3);
            System.out.println("总共有:" + set.size() + " 个元素!"); //结果：总共有:3 个元素! 
            p3.setAge(2); //修改p3的年龄,此时p3元素对应的hashcode值发生改变 
            set.remove(p3); //此时remove不掉，造成内存泄漏 
            set.add(p3); //重新添加，居然添加成功 
            System.out.println("总共有:" + set.size() + " 个元素!"); //结果：总共有:4 个元素! 
            for (Person person : set) {
                System.out.println(person);
            }
    
        }
    }
    
    
    public class Person {
        private String username;
        private String password;
        private int    age;
    
        public Person(String username, String password, int age) {
            this.username = username;
            this.password = password;
            this.age = age;
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        public String getPassword() {
            return password;
        }
    
        public void setPassword(String password) {
            this.password = password;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + age;
            result = prime * result + ((password == null) ? 0 : password.hashCode());
            result = prime * result + ((username == null) ? 0 : username.hashCode());
            return result;
        }
    
        @Override
        public boolean equals(Object obj) {
            if (this == obj)
                return true;
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;
            Person other = (Person) obj;
            if (age != other.age)
                return false;
            if (password == null) {
                if (other.password != null)
                    return false;
            } else if (!password.equals(other.password))
                return false;
            if (username == null) {
                if (other.username != null)
                    return false;
            } else if (!username.equals(other.username))
                return false;
            return true;
        }
    
        @Override
        public String toString() {
            return this.username + "-->" + this.password + "-->" + this.age;
        }
    
    }