Java使用Lombok详解

阳光穿透心脏的1/2处 2023-09-25 19:46 67阅读 0赞

#### 文章目录 ####

*  Lombok 快速入门
 *   *  Lombok 简介
     *  Lombok 安装
     *  Lombok 使用
     *   *  @Getter and @Setter
         *  @NonNull
         *  @ToString
         *  @EqualsAndHashCode
         *  @Data
         *  @Cleanup
         *  @Synchronized
         *  @SneakyThrows
     *  Lombok 使用注意点
     *   *  谨慎使用 \`@Builder\`
         *  \`@Data\` 注解和继承

## Lombok 快速入门 ##

### Lombok 简介 ###

Lombok 是一种 Java 实用工具，可用来帮助开发人员消除 Java 的冗长，尤其是对于简单的 Java 对象（POJO）。它通过注释实现这一目的。通过在开发环境中实现 Lombok，开发人员可以节省构建诸如 `hashCode()` 和 `equals()` 、`getter / setter` 这样的方法以及以往用来分类各种 accessor 和 mutator 的大量时间。

### Lombok 安装 ###

由于 Lombok 仅在编译阶段生成代码，所以使用 Lombok 注解的源代码，在 IDE 中会被高亮显示错误，针对这个问题可以通过安装 IDE 对应的插件来解决。具体的安装方式可以参考：[新版idea可略过][idea]

使 IntelliJ IDEA 支持 Lombok 方式如下：

*  **Intellij 设置支持注解处理**
    
     *  点击 File > Settings > Build > Annotation Processors
     *  勾选 Enable annotation processing
 *  **安装插件**
    
     *  点击 Settings > Plugins > Browse repositories
     *  查找 Lombok Plugin 并进行安装
     *  重启 IntelliJ IDEA
 *  **将 lombok 添加到 pom 文件**

<dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.16.8</version>
    </dependency>

### Lombok 使用 ###

Lombok 提供注解 API 来修饰指定的类：

#### @Getter and @Setter ####

[@Getter and @Setter][Getter and _Setter] Lombok 代码：

@Getter @Setter private boolean employed = true;
    @Setter(AccessLevel.PROTECTED) private String name;

等价于 Java 源码：

private boolean employed = true;
    private String name;
    
    public boolean isEmployed() {
        
        return employed;
    }
    
    public void setEmployed(final boolean employed) {
        
        this.employed = employed;
    }
    
    protected void setName(final String name) {
        
        this.name = name;
    }

#### @NonNull ####

[@NonNull][NonNull] Lombok 代码：

@Getter @Setter @NonNull
    private List<Person> members;

等价于 Java 源码：

@NonNull
    private List<Person> members;
    
    public Family(@NonNull final List<Person> members) {
        
        if (members == null) throw new java.lang.NullPointerException("members");
        this.members = members;
    }
    
    @NonNull
    public List<Person> getMembers() {
        
        return members;
    }
    
    public void setMembers(@NonNull final List<Person> members) {
        
        if (members == null) throw new java.lang.NullPointerException("members");
        this.members = members;
    }

#### @ToString ####

[@ToString][ToString] Lombok 代码：

@ToString(callSuper=true,exclude="someExcludedField")
    public class Foo extends Bar {
        
        private boolean someBoolean = true;
        private String someStringField;
        private float someExcludedField;
    }

等价于 Java 源码：

public class Foo extends Bar {
        
        private boolean someBoolean = true;
        private String someStringField;
        private float someExcludedField;
    
        @java.lang.Override
        public java.lang.String toString() {
        
            return "Foo(super=" + super.toString() +
                ", someBoolean=" + someBoolean +
                ", someStringField=" + someStringField + ")";
        }
    }

#### @EqualsAndHashCode ####

[@EqualsAndHashCode][EqualsAndHashCode] Lombok 代码：

@EqualsAndHashCode(callSuper=true,exclude={
        "address","city","state","zip"})
    public class Person extends SentientBeing {
        
        enum Gender {
         Male, Female }
    
        @NonNull private String name;
        @NonNull private Gender gender;
    
        private String ssn;
        private String address;
        private String city;
        private String state;
        private String zip;
    }

等价于 Java 源码：

public class Person extends SentientBeing {
        
    
        enum Gender {
        
            /*public static final*/ Male /* = new Gender() */,
            /*public static final*/ Female /* = new Gender() */;
        }
        @NonNull
        private String name;
        @NonNull
        private Gender gender;
        private String ssn;
        private String address;
        private String city;
        private String state;
        private String zip;
    
        @java.lang.Override
        public boolean equals(final java.lang.Object o) {
        
            if (o == this) return true;
            if (o == null) return false;
            if (o.getClass() != this.getClass()) return false;
            if (!super.equals(o)) return false;
            final Person other = (Person)o;
            if (this.name == null ? other.name != null : !this.name.equals(other.name)) return false;
            if (this.gender == null ? other.gender != null : !this.gender.equals(other.gender)) return false;
            if (this.ssn == null ? other.ssn != null : !this.ssn.equals(other.ssn)) return false;
            return true;
        }
    
        @java.lang.Override
        public int hashCode() {
        
            final int PRIME = 31;
            int result = 1;
            result = result * PRIME + super.hashCode();
            result = result * PRIME + (this.name == null ? 0 : this.name.hashCode());
            result = result * PRIME + (this.gender == null ? 0 : this.gender.hashCode());
            result = result * PRIME + (this.ssn == null ? 0 : this.ssn.hashCode());
            return result;
        }
    }

#### @Data ####

[@Data][Data] Lombok 代码：

@Data(staticConstructor="of")
    public class Company {
        
        private final Person founder;
        private String name;
        private List<Person> employees;
    }

@Data ：注解在类上；提供类所有属性的 `get` 和 `set` 方法，此外还提供了`equals`、`canEqual`、`hashCode`、`toString` 方法。  
等价于 Java 源码：

public class Company {
        
        private final Person founder;
        private String name;
        private List<Person> employees;
    
        private Company(final Person founder) {
        
            this.founder = founder;
        }
    
        public static Company of(final Person founder) {
        
            return new Company(founder);
        }
    
        public Person getFounder() {
        
            return founder;
        }
    
        public String getName() {
        
            return name;
        }
    
        public void setName(final String name) {
        
            this.name = name;
        }
    
        public List<Person> getEmployees() {
        
            return employees;
        }
    
        public void setEmployees(final List<Person> employees) {
        
            this.employees = employees;
        }
    
        @java.lang.Override
        public boolean equals(final java.lang.Object o) {
        
            if (o == this) return true;
            if (o == null) return false;
            if (o.getClass() != this.getClass()) return false;
            final Company other = (Company)o;
            if (this.founder == null ? other.founder != null : !this.founder.equals(other.founder)) return false;
            if (this.name == null ? other.name != null : !this.name.equals(other.name)) return false;
            if (this.employees == null ? other.employees != null : !this.employees.equals(other.employees)) return false;
            return true;
        }
    
        @java.lang.Override
        public int hashCode() {
        
            final int PRIME = 31;
            int result = 1;
            result = result * PRIME + (this.founder == null ? 0 : this.founder.hashCode());
            result = result * PRIME + (this.name == null ? 0 : this.name.hashCode());
            result = result * PRIME + (this.employees == null ? 0 : this.employees.hashCode());
            return result;
        }
    
        @java.lang.Override
        public java.lang.String toString() {
        
            return "Company(founder=" + founder + ", name=" + name + ", employees=" + employees + ")";
        }
    }

#### @Cleanup ####

[@Cleanup][Cleanup] Lombok 代码：

public void testCleanUp() {
        
        try {
        
            @Cleanup ByteArrayOutputStream baos = new ByteArrayOutputStream();
            baos.write(new byte[] {
        'Y','e','s'});
            System.out.println(baos.toString());
        } catch (IOException e) {
        
            e.printStackTrace();
        }
    }

`@Cleanup`：作用于变量，自动关闭资源，仅针对实现了 `java.io.Closeable` 接口的对象有效。  
等价于 Java 源码：

public void testCleanUp() {
        
        try {
        
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            try {
        
                baos.write(new byte[]{
        'Y', 'e', 's'});
                System.out.println(baos.toString());
            } finally {
        
                baos.close();
            }
        } catch (IOException e) {
        
            e.printStackTrace();
        }
    }

#### @Synchronized ####

[@Synchronized][Synchronized] Lombok 代码：

private DateFormat format = new SimpleDateFormat("MM-dd-YYYY");
    
    @Synchronized
    public String synchronizedFormat(Date date) {
        
        return format.format(date);
    }

`@Synchronized`：作用于方法，可以替换 synchronized 关键字或 lock 锁。  
等价于 Java 源码：

private final java.lang.Object $lock = new java.lang.Object[0];
    private DateFormat format = new SimpleDateFormat("MM-dd-YYYY");
    
    public String synchronizedFormat(Date date) {
        
        synchronized ($lock) {
        
            return format.format(date);
        }
    }

#### @SneakyThrows ####

[@SneakyThrows][SneakyThrows] Lombok 代码：

@SneakyThrows
    public void testSneakyThrows() {
        
        throw new IllegalAccessException();
    }

`@SneakyThrows`：作用于方法，对异常进行捕捉并抛出。  
等价于 Java 源码：

public void testSneakyThrows() {
        
        try {
        
            throw new IllegalAccessException();
        } catch (java.lang.Throwable $ex) {
        
            throw lombok.Lombok.sneakyThrow($ex);
        }
    }

### Lombok 使用注意点 ###

#### 谨慎使用 `@Builder` ####

在类上标注了 `@Data` 和 `@Builder` 注解的时候，编译时，lombok 优化后的 Class 中会没有默认的构造方法。在反序列化的时候，没有默认构造方法就可能会报错。

【示例】使用 `@Builder` 不当导致 json 反序列化失败

@Data
    @Builder
    public class BuilderDemo01 {
        
    
        private String name;
    
        public static void main(String[] args) throws JsonProcessingException {
        
            BuilderDemo01 demo01 = BuilderDemo01.builder().name("demo01").build();
            ObjectMapper mapper = new ObjectMapper();
            String json = mapper.writeValueAsString(demo01);
            BuilderDemo01 expectDemo01 = mapper.readValue(json, BuilderDemo01.class);
            System.out.println(expectDemo01.toString());
        }
    
    }

运行时会抛出异常：

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `io.github.dunwu.javatech.bean.lombok.BuilderDemo01` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
     at [Source: (String)"{"name":"demo01"}"; line: 1, column: 2]
    	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
    	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1432)
    	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1062)
    	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297)
    	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
    	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
    	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4218)
    	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3214)
    	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3182)
    	at io.github.dunwu.javatech.bean.lombok.BuilderDemo01.main(BuilderDemo01.java:22)

【示例】使用 `@Builder` 正确方法

@Data
    @Builder
    @NoArgsConstructor
    @AllArgsConstructor
    public class BuilderDemo02 {
        
    
        private String name;
    
        public static void main(String[] args) throws JsonProcessingException {
        
            BuilderDemo02 demo02 = BuilderDemo02.builder().name("demo01").build();
            ObjectMapper mapper = new ObjectMapper();
            String json = mapper.writeValueAsString(demo02);
            BuilderDemo02 expectDemo02 = mapper.readValue(json, BuilderDemo02.class);
            System.out.println(expectDemo02.toString());
        }
    
    }

#### `@Data` 注解和继承 ####

使用 `@Data` 注解时，则有了 `@EqualsAndHashCode` 注解，那么就会在此类中存在 `equals(Object other)` 和 `hashCode()` 方法，且不会使用父类的属性，这就导致了可能的问题。比如，有多个类有相同的部分属性，把它们定义到父类中，恰好 id（数据库主键）也在父类中，那么就会存在部分对象在比较时，它们并不相等，这是因为：lombok 自动生成的 `equals(Object other)` 和 `hashCode()` 方法判定为相等，从而导致和预期不符。

修复此问题的方法很简单：

*  使用 `@Data` 时，加上 `@EqualsAndHashCode(callSuper=true)` 注解。
 *  使用 `@Getter @Setter @ToString` 代替 `@Data` 并且自定义 `equals(Object other)` 和 `hashCode()` 方法。

【示例】测试 `@Data` 和 `@EqualsAndHashCode`

@Data
    @ToString(exclude = "age")
    @EqualsAndHashCode(exclude = {
         "age", "sex" })
    public class Person {
        
    
        protected String name;
    
        protected Integer age;
    
        protected String sex;
    
    }
    
    @Data
    @EqualsAndHashCode(callSuper = true, exclude = {
         "address", "city", "state", "zip" })
    public class EqualsAndHashCodeDemo extends Person {
        
    
        @NonNull
        private String name;
    
        @NonNull
        private Gender gender;
    
        private String ssn;
    
        private String address;
    
        private String city;
    
        private String state;
    
        private String zip;
    
        public EqualsAndHashCodeDemo(@NonNull String name, @NonNull Gender gender) {
        
            this.name = name;
            this.gender = gender;
        }
    
        public EqualsAndHashCodeDemo(@NonNull String name, @NonNull Gender gender,
            String ssn, String address, String city, String state, String zip) {
        
            this.name = name;
            this.gender = gender;
            this.ssn = ssn;
            this.address = address;
            this.city = city;
            this.state = state;
            this.zip = zip;
        }
    
        public enum Gender {
        
            Male,
            Female
        }
    
    }
    
    @Test
    @DisplayName("测试 @EqualsAndHashCode")
    public void testEqualsAndHashCodeDemo() {
        
        EqualsAndHashCodeDemo demo1 =
            new EqualsAndHashCodeDemo("name1", EqualsAndHashCodeDemo.Gender.Female, "ssn", "xxx", "xxx", "xxx", "xxx");
        EqualsAndHashCodeDemo demo2 =
            new EqualsAndHashCodeDemo("name1", EqualsAndHashCodeDemo.Gender.Female, "ssn", "ooo", "ooo", "ooo", "ooo");
        Assertions.assertEquals(demo1, demo2);
    
        Person person = new Person();
        person.setName("张三");
        person.setAge(20);
        person.setSex("男");
    
        Person person2 = new Person();
        person2.setName("张三");
        person2.setAge(18);
        person2.setSex("男");
    
        Person person3 = new Person();
        person3.setName("李四");
        person3.setAge(20);
        person3.setSex("男");
    
        Assertions.assertEquals(person2, person);
        Assertions.assertNotEquals(person3, person);
    }

上面的单元测试可以通过，但如果将 `@EqualsAndHashCode(callSuper = true, exclude = { "address", "city", "state", "zip" })` 注掉就会报错。

[idea]: https://www.baeldung.com/lombok-ide
[Getter and _Setter]: http://jnb.ociweb.com/jnb/jnbJan2010.html#gettersetter
[NonNull]: http://jnb.ociweb.com/jnb/jnbJan2010.html#nonnull
[ToString]: http://jnb.ociweb.com/jnb/jnbJan2010.html#tostring
[EqualsAndHashCode]: http://jnb.ociweb.com/jnb/jnbJan2010.html#equals
[Data]: http://jnb.ociweb.com/jnb/jnbJan2010.html#data
[Cleanup]: http://jnb.ociweb.com/jnb/jnbJan2010.html#cleanup
[Synchronized]: http://jnb.ociweb.com/jnb/jnbJan2010.html#synchronized
[SneakyThrows]: http://jnb.ociweb.com/jnb/jnbJan2010.html#sneaky