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()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

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 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 Lombok 代码:

@Getter @Setter @NonNullprivate List<Person> members;

等价于 Java 源码:

@NonNullprivate List<Person> members;public Family(@NonNull final List<Person> members) {    if (members == null) throw new java.lang.NullPointerException("members");    this.members = members;}@NonNullpublic 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 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 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 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 getset 方法,此外还提供了equalscanEqualhashCodetoString 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 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 Lombok 代码:

private DateFormat format = new SimpleDateFormat("MM-dd-YYYY");@Synchronizedpublic 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 Lombok 代码:

@SneakyThrowspublic 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@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@Builderpublic 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@AllArgsConstructorpublic 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)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)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)hashCode() 方法。

【示例】测试 @Data@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。