被SpringBoot和IDEA支持的Lombok到底有何神奇之处

最近 IDEA 2020 最后一个版本发布了，听说已经内置了 Lombok 插件，SpringBoot 2.1.x 之后的版本也在 Starter 中内置了 Lombok 依赖

Lombok简介

Lombok 是一款 Java 代码功能增强库，在 Github 上已有 9.9 k Star。它会自动集成到你的编辑器和构建工具中，从而使你的 Java 代码更加生动有趣。通过 Lombok 的注解，你可以不用再写 getter、setter、equals 等方法，Lombok 将在编译时为你自动生成。

Lombok安装

首先我们需要在 IDEA 中安装好 Lombok 插件，如果你使用的是最新版 IDEA 2020.3，则 Lombok 插件已经内置，无需安装。

之后在项目的 pom.xml 文件中添加 Lombok 依赖就好啦

SpringBoot 2.1.x 版本后无需指定 Lombok 版本，因为 SpringBoot 在 spring-boot-dependencies 中已经内置了 Lombok

Lombok使用

Lombok 中有很多注解，这些注解使得我们可以更加方便的编写 Java 代码，下面介绍下这些注解的使用。

val

使用 val 注解可以取代任意类型作为局部变量，这样我们就不用写复杂的 ArrayList 和 Map.Entry 类型了，具体例子如下。

/**
 * @author https://blog.imzjw.cn
 * @date 2020/12/23 20:03
 */
public class LombokDemo {

    public static void main(String[] args) {
        result();
        result2();
    }

    private static void result() {
        // val代替 ArrayList<String> 和 String 类型
        val list = new ArrayList<String>();
        list.add("GARVEY ZHONG");
        val res = list.get(0);
        // toLowerCase：转小写
        System.out.println(res.toLowerCase());
    }

    private static void result2() {
        // val 代替 Map.Entry<Integer,String> 类型
        val map = new HashMap<Integer, String>();
        map.put(0, "blog.imzjw.cn");
        System.out.println(map.get(0));
    }
}

// 打印结果
===========================================================
"C:\Program Files\Java\jdk1.8.0_162\bin\java.exe"...		
garvey zhong												
blog.imzjw.cn											

Process finished with exit code 0
===========================================================

当我们使用了 val 注解后，Lombok 会从局部变量的初始化表达式推断出具体类型，编译后会生成如下代码。

public class LombokDemo {
    public LombokDemo() {
    }

    public static void result() {
        ArrayList<String> list = new ArrayList<>();
        list.add("GARVEY ZHONG");
        String res = list.get(0);
        System.out.println(res.toLowerCase());
    }

    public static void result2() {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(0, "blog.imzjw.cn");
        System.out.println(map.get(0));
    }
}

@NonNull

在方法上使用 @NonNull 注解可以做非空判断，如果传入空值的话会直接抛出 NullPointerException （俗称空指针）

public class LombokDemo {

    private String name;
    
    public static void main(String[] args) {
        new LombokDemo("garvey");
        // 绝对会抛出NullPointerException异常
        new LombokDemo(null);
    }

    public LombokDemo(@NonNull String name) {
        this.name = name;
    }
}

编译后会在构造器中添加非空判断，具体代码如下。

public class LombokDemo {

    private String name;

    public static void main(String[] args) {
        new LombokDemo("garvey");
        new LombokDemo((String)null);
    }

    public LombokDemo(@NonNull String name) {
        if (name == null) {
            throw new NullPointerException("name is marked non-null but is null");
        } else {
            this.name = name;
        }
    }
}

@Cleanup

当我们在 Java 中使用资源时，不可避免地需要在使用后关闭资源。使用 @Cleanup 注解可自动关闭资源。

public static void main(String[] args) throws IOException {
    String str = "garvey";
    // 使用输入输出流自动关闭，无需编写 try catch 和调用 close() 方法
    @Cleanup ByteArrayInputStream in = new ByteArrayInputStream(str.getBytes("UTF-8"));
    @Cleanup ByteArrayOutputStream out = new ByteArrayOutputStream();
    byte[] b = new byte[1024];
    while (true) {
        int r = in.read(b);
        if (r == -1) break;
        out.write(b, 0, r);
    }
    System.out.println(out.toString("UTF-8"));
}

编译后 Lombok 会生成如下代码。

public static void main(String[] args) throws IOException {
    String str = "garvey";
    ByteArrayInputStream in = new ByteArrayInputStream(inStr.getBytes("UTF-8"));

    try {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        try {
            byte[] b = new byte[1024];

            while (true) {
                int r = in.read(b);
                if (r == -1) {
                    System.out.println(out.toString("UTF-8"););
                    return;
                }
                out.write(b, 0, r);
            }
        } finally {
            if (Collections.singletonList(out).get(0) != null) {
                out.close();
            }
        }
    } finally {
        if (Collections.singletonList(in).get(0) != null) {
            in.close();
        }
    }
}

@Getter/@Setter

有了 @Getter 和 @Setter 注解，我们再也不用编写 getter and setter 方法了。

其实写 getter 和 setter 方法也不是麻烦事，因为 IDEA 自动生成 getter and setter 方法

麻烦的是如果类属性的类型和名称改了的话，又要重新生成 getter and setter 方法，这就很头疼

public class Student {
    @Getter
    @Setter
    private String name;
    @Getter
    @Setter(AccessLevel.PROTECTED)
    private Integer age;

    public static void main(String[] args) {
        Student student = new Student();
        student.setName("garvey");
        student.setAge(20);
        System.out.println("name=" + student.getName() + "\nage=" + student.getAge());
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    private String name;
    private Integer age;

    public Student () {
    }

    public String getName() {
        return this.name;
    }

    public void setName(final String name) {
        this.name = name;
    }

    public Integer getAge() {
        return this.age;
    }

    protected void setAge(final Integer age) {
        this.age = age;
    }
}

@ToString

使用 @ToString 注解可以自动生成 toString 方法，默认会包含所有类属性，使用 @ToString.Exclude 注解可以排除属性的生成。

@ToString
public class Student {

    @ToString.Exclude
    private Long id;
    private String name;
    private Integer age;

    public Student(Long id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public static void main(String[] args) {
        Student student = new Student(1L, "garvey", 20);
        // 自动实现toString方法，输出ToStringExample(name=garvey, age=20)
        System.out.println(student);
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    private Long id;
    private String name;
    private Integer age;

    public Student (Long id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public String toString() {
        return "Student(name=" + this.name + ", age=" + this.age + ")";
    }
}

@EqualsAndHashCode

使用 @EqualsAndHashCode 注解可以自动生成 hashCode 和 equals 方法，默认包含所有类属性，使用 @EqualsAndHashCode.Exclude可以排除属性的生成。

@Getter
@Setter
@EqualsAndHashCode
public class Student {

    private Long id;
    @EqualsAndHashCode.Exclude
    private String name;

    @EqualsAndHashCode.Exclude
    private Integer age;

    public static void main(String[] args) {
        Student student = new Student();
        student.setId(1L);
        student.setName("garvey");
        student.setAge(20);
        Student student2 = new Student();
        student2.setId(1L);
        // equals方法只对比id，so 返回true
        System.out.println(student.equals(student2));
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    private Long id;
    private String name;
    private Integer age;

    public Student() {
    }

    public boolean equals(final Object o) {
        if (o == this) {
            return true;
        } else if (!(o instanceof Student)) {
            return false;
        } else {
            Student other = (Student)o;
            if (!other.canEqual(this)) {
                return false;
            } else {
                Object this$id = this.getId();
                Object other$id = other.getId();
                if (this$id == null) {
                    if (other$id != null) {
                        return false;
                    }
                } else if (!this$id.equals(other$id)) {
                    return false;
                }

                return true;
            }
        }
    }

    protected boolean canEqual(final Object other) {
        return other instanceof Student;
    }

    public int hashCode() {
        int PRIME = true;
        int result = 1;
        Object $id = this.getId();
        int result = result * 59 + ($id == null ? 43 : $id.hashCode());
        return result;
    }
}

@XxConstructor

使用 @XxConstructor 注解可以自动生成构造方法，有 @NoArgsConstructor、 @RequiredArgsConstructor 和 @AllArgsConstructor 三个注解可以使用。

• @NoArgsConstructor：生成无参构造函数。
• @RequiredArgsConstructor：生成包含必须参数的构造函数，使用 @NonNull 注解的类属性为必须参数。
• @AllArgsConstructor：生成包含所有参数的构造函数。

@NoArgsConstructor
@RequiredArgsConstructor(staticName = "of")
@AllArgsConstructor
public class Student {
    @NonNull
    private Long id;
    private String name;
    private Integer age;

    public static void main(String[] args) {
        // 无参构造器
        Student student1 = new Student();
        // 全部参数构造器
        Student student2 = new Student(1L, "garvey", 20);
        // @NonNull注解的必须参数构造器
        Student student3 = Student.of(1L);
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    @NonNull
    private Long id;
    private String name;
    private Integer age;

    public Student() {
    }

    private Student(@NonNull final Long id) {
        if (id == null) {
            throw new NullPointerException("id is marked non-null but is null");
        } else {
            this.id = id;
        }
    }

    public static Student of(@NonNull final Long id) {
        return new Student(id);
    }

    public Student(@NonNull final Long id, final String name, final Integer age) {
        if (id == null) {
            throw new NullPointerException("id is marked non-null but is null");
        } else {
            this.id = id;
            this.name = name;
            this.age = age;
        }
    }
}

@Data

@Data 是一个方便使用的组合注解，是 @ToString、@EqualsAndHashCode、@Getter、@Setter 和 @RequiredArgsConstructor 的组合体。

@Data
public class Student {
    @NonNull
    private Long id;
    @EqualsAndHashCode.Exclude
    private String name;
    @EqualsAndHashCode.Exclude
    private Integer age;

    public static void main(String[] args) {
        // @RequiredArgsConstructor已生效
        Student student1 = new Student(1L);
        // @Getter @Setter已生效
        student1.setName("garvey");
        student1.setAge(20);
        // @ToString已生效
        System.out.println(student1);
        Student student2 = new Student(1L);
        // @EqualsAndHashCode已生效
        System.out.println(student1.equals(student2));
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    @NonNull
    private Long id;
    private String name;
    private Integer age;

    public Student(@NonNull final Long id) {
        if (id == null) {
            throw new NullPointerException("id is marked non-null but is null");
        } else {
            this.id = id;
        }
    }

    @NonNull
    public Long getId() {
        return this.id;
    }

    public String getName() {
        return this.name;
    }

    public Integer getAge() {
        return this.age;
    }

    public void setId(@NonNull final Long id) {
        if (id == null) {
            throw new NullPointerException("id is marked non-null but is null");
        } else {
            this.id = id;
        }
    }

    public void setName(final String name) {
        this.name = name;
    }

    public void setAge(final Integer age) {
        this.age = age;
    }

    public boolean equals(final Object o) {
        if (o == this) {
            return true;
        } else if (!(o instanceof Student)) {
            return false;
        } else {
            Student other = (Student)o;
            if (!other.canEqual(this)) {
                return false;
            } else {
                Object this$id = this.getId();
                Object other$id = other.getId();
                if (this$id == null) {
                    if (other$id != null) {
                        return false;
                    }
                } else if (!this$id.equals(other$id)) {
                    return false;
                }

                return true;
            }
        }
    }

    protected boolean canEqual(final Object other) {
        return other instanceof Student;
    }

    public int hashCode() {
        int PRIME = true;
        int result = 1;
        Object $id = this.getId();
        int result = result * 59 + ($id == null ? 43 : $id.hashCode());
        return result;
    }

    public String toString() {
        return "Student(id=" + this.getId() + ", name=" + this.getName() + ", age=" + this.getAge() + ")";
    }
}

@Value

使用 @Value 注解可以把类声明为不可变的，声明后此类相当于 final 类，无法被继承，其属性也会变成 final 属性。

@Value
public class Student {
    private Long id;
    private String name;
    private Integer age;

    public static void main(String[] args) {
        // 只能使用全参构造器
        Student student = new Student(1L,"garvey",20);
        // student.setName("garvey") // 没有生成setter方法，会报错
        // student.name="garvey"    // 字段被设置为final类型，会报错
    }
}

编译后 Lombok 会生成如下代码。

public final class Student {
    private final Long id;
    private final String name;
    private final Integer age;

    public static void main(String[] args) {
        new Student(1L, "garvey", 20);
    }

    public Student(final Long id, final String name, final Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public Long getId() {
        return this.id;
    }

    public String getName() {
        return this.name;
    }

    public Integer getAge() {
        return this.age;
    }
}

@Builder

使用 @Builder 注解可以通过建造者模式来创建对象，建造者模式加链式调用，创建对象太方便了！

@Builder
@ToString
public class Student {
	private Long id;
	private String name;
	private Integer age;

	public static void main(String[] args) {
		Student student = Student.builder()
				.id(1L)
				.name("garvey")
				.age(20)
				.build();
		System.out.println(student);
	}
}

编译后 Lombok 会生成如下代码。

public class Student {
	private Long id;
	private String name;
	private Integer age;

	Student(final Long id, final String name, final Integer age) {
		this.id = id;
		this.name = name;
		this.age = age;
	}

	public static Student.BuilderExampleBuilder builder() {
		return new Student.BuilderExampleBuilder();
	}

	public String toString() {
		return "Student(id=" + this.id + ", name=" + this.name + ", age=" + this.age + ")";
	}

	public static class BuilderExampleBuilder {
		private Long id;
		private String name;
		private Integer age;

		BuilderExampleBuilder() {
		}

		public Student.BuilderExampleBuilder id(final Long id) {
			this.id = id;
			return this;
		}

		public Student.BuilderExampleBuilder name(final String name) {
			this.name = name;
			return this;
		}

		public Student.BuilderExampleBuilder age(final Integer age) {
			this.age = age;
			return this;
		}

		public Student build() {
			return new Student(this.id, this.name, this.age);
		}

		public String toString() {
			return "Student.BuilderExampleBuilder(id=" + this.id + ", name=" + this.name + ", age=" + this.age + ")";
		}
	}
}

@SneakyThrows

还在手动捕获并抛出异常？使用 @SneakyThrows 注解自动实现试试！

public class Student {

    // 自动抛出异常，无需处理
    @SneakyThrows(UnsupportedEncodingException.class)
    public static byte[] str2byte(String str) {
        return str.getBytes("UTF-8");
    }

    public static void main(String[] args) {
        String str = "garvey";
        System.out.println(str2byte(str).length);
    }
}

编译后 Lombok 会生成如下代码。

public class Student {
    public Student() {
    }

    public static byte[] str2byte(String str) {
        try {
            return str.getBytes("UTF-8");
        } catch (UnsupportedEncodingException var2) {
            throw var2;
        }
    }
}

@Synchronized

当我们在多个线程中访问同一资源时，往往会出现线程安全问题，以前我们往往使用 synchronized 关键字修饰方法来实现同步访问。使用 @Synchronized 注解同样可以实现同步访问。

@Data
public class SynchronizedExample {
    @NonNull
    private Integer count;

    @Synchronized
    @SneakyThrows
    public void reduceCount(Integer id) {
        if (count > 0) {
            Thread.sleep(500);
            count--;
            System.out.println(String.format("thread-%d count:%d", id, count));
        }
    }

    public static void main(String[] args) {
        // 添加@Synchronized三个线程可以同步调用reduceCount方法
        SynchronizedExample example = new SynchronizedExample(20);
        new ReduceThread(1, example).start();
        new ReduceThread(2, example).start();
        new ReduceThread(3, example).start();
    }


    @RequiredArgsConstructor
    static class ReduceThread extends Thread {
        @NonNull
        private Integer id;
        @NonNull
        private SynchronizedExample example;

        @Override
        public void run() {
            while (example.getCount() > 0) {
                example.reduceCount(id);
            }
        }
    }
}

编译后 Lombok 会生成如下代码。

public class SynchronizedExample {
    private final Object $lock = new Object[0];
    @NonNull
    private Integer count;

    public void reduceCount(Integer id) {
        try {
            synchronized(this.$lock) {
                if (this.count > 0) {
                    Thread.sleep(500L);
                    Integer var3 = this.count;
                    Integer var4 = this.count = this.count - 1;
                    System.out.println(String.format("thread-%d count:%d", id, this.count));
                }

            }
        } catch (Throwable var7) {
            throw var7;
        }
    }
}

@With

使用 @With 注解可以实现对原对象进行克隆，并改变其一个属性，使用时需要指定全参构造方法。

@With
@AllArgsConstructor
public class WithExample {
    private Long id;
    private String name;
    private Integer age;

    public static void main(String[] args) {
        WithExample example1 = new WithExample(1L, "garvey", 20);
        WithExample example2 = example1.withAge(22);
        // 将原对象进行clone并设置age，返回false
        System.out.println(example1.equals(example2));
    }
}

编译后 Lombok 会生成如下代码。

public class WithExample {
    private Long id;
    private String name;
    private Integer age;

    public WithExample withId(final Long id) {
        return this.id == id ? this : new WithExample(id, this.name, this.age);
    }

    public WithExample withName(final String name) {
        return this.name == name ? this : new WithExample(this.id, name, this.age);
    }

    public WithExample withAge(final Integer age) {
        return this.age == age ? this : new WithExample(this.id, this.name, age);
    }

    public WithExample(final Long id, final String name, final Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
}

@Getter(lazy=true)

当我们获取某一个属性比较消耗资源时，可以给 @Getter 添加 lazy=true 属性实现懒加载，会生成 Double Check Lock 样板代码对属性进行懒加载。

public class GetterLazyExample {
    @Getter(lazy = true)
    private final double[] cached = expensive();

    private double[] expensive() {
        double[] result = new double[1000000];
        for (int i = 0; i < result.length; i++) {
            result[i] = Math.asin(i);
        }
        return result;
    }

    public static void main(String[] args) {
        // 使用Double Check Lock 样板代码对属性进行懒加载
        GetterLazyExample example = new GetterLazyExample();
        System.out.println(example.getCached().length);
    }
}

编译后 Lombok 会生成如下代码。

public class GetterLazyExample {
    private final AtomicReference<Object> cached = new AtomicReference();

    public GetterLazyExample() {
    }

    private double[] expensive() {
        double[] result = new double[1000000];

        for(int i = 0; i < result.length; ++i) {
            result[i] = Math.asin((double)i);
        }
        return result;
    }

    public double[] getCached() {
        Object value = this.cached.get();
        if (value == null) {
            synchronized(this.cached) {
                value = this.cached.get();
                if (value == null) {
                    double[] actualValue = this.expensive();
                    value = actualValue == null ? this.cached : actualValue;
                    this.cached.set(value);
                }
            }
        }

        return (double[])((double[])(value == this.cached ? null : value));
    }
}

@Log

使用 @Log 注解，可以直接生成日志对象 log，通过 log 对象可以直接打印日志。

@Log
public class LogExample {
    public static void main(String[] args) {
        log.info("level info");
        log.warning("level warning");
        log.severe("level severe");
    }
}

编译后 Lombok 会生成如下代码。

public class LogExample {
    private static final Logger log = Logger.getLogger(LogExample.class.getName());

    public LogExample() {
    }

    public static void main(String[] args) {
        log.info("level info");
        log.warning("level warning");
        log.severe("level severe");
    }
}

@Slf4j

使用 Lombok 生成日志对象时，根据使用日志实现的不同，有多种注解可以使用。比如 @Log、@Log4j、@Log4j2、@Slf4j 等。

@Slf4j
public class LogSlf4jExample {
    public static void main(String[] args) {
        log.info("level:{}","info");
        log.warn("level:{}","warn");
        log.error("level:{}", "error");
    }
}

编译后 Lombok 会生成如下代码。

public class LogSlf4jExample {
    private static final Logger log = LoggerFactory.getLogger(LogSlf4jExample.class);

    public LogSlf4jExample() {
    }

    public static void main(String[] args) {
        log.info("level:{}", "info");
        log.warn("level:{}", "warn");
        log.error("level:{}", "error");
    }
}

参考资料

• 官方文档：https://projectlombok.org/features/all
• macrozheng