Lombok引入简化Java代码

转载 http://t.cn/RS0UdrX

Lombok简介

如Github上项目介绍所言，Lombok项目通过添加“处理程序”，使java成为一种更为简单的语言。作为一个Old Java Developer,我们都知道我们经常需要定义一系列的套路，比如定义如下的格式对象。

public class DataExample {

  private final String name;

  private int age;

  private double score;

  private String[] tags;

}

我们往往需要定义一系列的Get和Set方法最终展示形式如：

public class DataExample {

  private final String name;

  private int age;

  private double score;

  private String[] tags;

  public DataExample(String name) {
    this.name = name;
  }
  public String getName() {
    return this.name;
  }
  void setAge(int age) {
    this.age = age;
  }
  public int getAge() {
    return this.age;
  }
  public void setScore(double score) {
    this.score = score;
  }
  public double getScore() {
    return this.score;
  }
  public String[] getTags() {
    return this.tags;
  }
  public void setTags(String[] tags) {
    this.tags = tags;
  }
}

那我们有没有可以简化的办法呢，第一种就是使用IDEA等IDE提供的一键生成的快捷键，第二种就是我们今天介绍的 Lombok项目：

@Data 
public class DataExample {
  private final String name;
  @Setter(AccessLevel.PACKAGE) 
  private int age;
  private double score;
  private String[] tags;
}

Wow...这样就可以完成我们的需求，简直是太棒了，仅仅需要几个注解，我们就拥有了完整的GetSet方法，还包含了ToString等方法的生成。

几种常用的 lombok 注解：
@Data   ：注解在类上；提供类所有属性的 getting 和 setting 方法，此外还提供了equals、canEqual、hashCode、toString 方法
@Setter：注解在属性上；为属性提供 setting 方法
@Getter：注解在属性上；为属性提供 getting 方法
@Log4j ：注解在类上；为类提供一个 属性名为log 的 log4j 日志对象
@NoArgsConstructor：注解在类上；为类提供一个无参的构造方法
@AllArgsConstructor：注解在类上；为类提供一个全参的构造方法
@RequiredArgsConstructor： 会生成一个包含常量，和标识了NotNull的变量 的构造方法。生成的构造方法是private，如何想要对外提供使用可以使用staticName选项生成一个static方法。

Lombok安装

<dependency>
     <groupId>org.projectlombok</groupId>
     <artifactId>lombok</artifactId>
     <version>${lombok.version}</version>
     <scope>provided</scope>
 </dependency>

编辑器需要安装插件，intellij idea 安装插件 Lombok Plugin

Lombok使用

1. val

如果对其他的语言有研究的会发现，很多语言是使用 var 作为变量申明，val作为常量申明。这里的val也是这个作用。

public String example() {
    val example = new ArrayList<String>();
    example.add("Hello, World!");
    val foo = example.get(0);
    return foo.toLowerCase();
}

翻译成 Java 程序是：

public String example() {
    final ArrayList<String> example = new ArrayList<String>();
    example.add("Hello, World!");
    final String foo = example.get(0);
    return foo.toLowerCase();
}

2. @NonNull

使用 @NonNull 注解修饰的字段 通过 set 方法设置时如果为 null, 将抛出 NullPointerException

public class NonNullExample extends Something {
  private String name;
  
  public NonNullExample(@NonNull Person person) {
    super("Hello");
    this.name = person.getName();
  }
}

翻译成 Java 程序是：

public class NonNullExample extends Something {
  private String name;
  
  public NonNullExample(@NonNull Person person) {
    super("Hello");
    if (person == null) {
      throw new NullPointerException("person");
    }
    this.name = person.getName();
  }
}

3. @Cleanup

主要用来修饰 IO 流相关类, 会在 finally 代码块中对该资源进行 close();

public class CleanupExample {
  public static void main(String[] args) throws IOException {
    @Cleanup InputStream in = new FileInputStream(args[0]);
    @Cleanup OutputStream out = new FileOutputStream(args[1]);
    byte[] b = new byte[10000];
    while (true) {
      int r = in.read(b);
      if (r == -1) break;
      out.write(b, 0, r);
    }
  }
}

翻译成 Java 程序是：

public class CleanupExample {
  public static void main(String[] args) throws IOException {
    InputStream in = new FileInputStream(args[0]);
    try {
      OutputStream out = new FileOutputStream(args[1]);
      try {
        byte[] b = new byte[10000];
        while (true) {
          int r = in.read(b);
          if (r == -1) break;
          out.write(b, 0, r);
        }
      } finally {
        if (out != null) {
          out.close();
        }
      }
    } finally {
      if (in != null) {
        in.close();
      }
    }
  }
}

4. @Getter/@Setter

注解在属性上；为属性提供 setting、getting 方法

public class GetterSetterExample {

  @Getter @Setter private int age = 10;
  
  @Setter(AccessLevel.PROTECTED) private String name;
  
  @Override public String toString() {
    return String.format("%s (age: %d)", name, age);
  }
}

翻译成 Java 程序是：

public class GetterSetterExample {

  private int age = 10;

  private String name;
  
  @Override public String toString() {
    return String.format("%s (age: %d)", name, age);
  }
  
  public int getAge() {
    return age;
  }
  
  public void setAge(int age) {
    this.age = age;
  }
  
  protected void setName(String name) {
    this.name = name;
  }
}

5. @ToString

@ToString(exclude="id")
public class ToStringExample {
  private static final int STATIC_VAR = 10;
  private String name;
  private Shape shape = new Square(5, 10);
  private String[] tags;
  private int id;
  
  public String getName() {
    return this.getName();
  }
  
  @ToString(callSuper=true, includeFieldNames=true)
  public static class Square extends Shape {
    private final int width, height;
    
    public Square(int width, int height) {
      this.width = width;
      this.height = height;
    }
  }
}

编译后

public class ToStringExample {
  private static final int STATIC_VAR = 10;
  private String name;
  private Shape shape = new Square(5, 10);
  private String[] tags;
  private int id;
  
  public String getName() {
    return this.getName();
  }
  
  public static class Square extends Shape {
    private final int width, height;
    
    public Square(int width, int height) {
      this.width = width;
      this.height = height;
    }
    
    @Override public String toString() {
      return "Square(super=" + super.toString() + ", width=" + this.width + ", height=" + this.height + ")";
    }
  }
  
  @Override public String toString() {
    return "ToStringExample(" + this.getName() + ", " + this.shape + ", " + Arrays.deepToString(this.tags) + ")";
  }
}

6. @NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor

@NoArgsConstructor：注解在类上；为类提供一个无参的构造方法
@AllArgsConstructor：注解在类上；为类提供一个全参的构造方法
@RequiredArgsConstructor 为未初始化的 final 字段和使用 @NonNull 标注的字段生成构造函数

@RequiredArgsConstructor(staticName = "of")
@AllArgsConstructor(access = AccessLevel.PROTECTED)
public class ConstructorExample<T> {
  private int x, y;
  @NonNull private T description;
  
  @NoArgsConstructor
  public static class NoArgsExample {
    @NonNull private String field;
  }
}

编译后

public class ConstructorExample<T> {
  private int x, y;
  @NonNull private T description;
  
  private ConstructorExample(T description) {
    if (description == null) throw new NullPointerException("description");
    this.description = description;
  }
  
  public static <T> ConstructorExample<T> of(T description) {
    return new ConstructorExample<T>(description);
  }
  
  @java.beans.ConstructorProperties({"x", "y", "description"})
  protected ConstructorExample(int x, int y, T description) {
    if (description == null) throw new NullPointerException("description");
    this.x = x;
    this.y = y;
    this.description = description;
  }
  
  public static class NoArgsExample {
    @NonNull private String field;
    
    public NoArgsExample() {
    }
  }
}

7. @Data

这个就相当的简单啦，因为我们发现 @ToString, @EqualsAndHashCode, @Getter 都很常用，这个一个注解就相当于 

@ToString, @EqualsAndHashCode, @Getter(所有字段), @Setter (所有非final字段), @RequiredArgsConstructor!

8. @Value

 @Value 就是 @Data 的不可变版本

@Value public class ValueExample {
  String name;
  @Wither(AccessLevel.PACKAGE) @NonFinal int age;
  double score;
  protected String[] tags;
  
  @ToString(includeFieldNames=true)
  @Value(staticConstructor="of")
  public static class Exercise<T> {
    String name;
    T value;
  }
}

编译后

public final class ValueExample {
  private final String name;
  private int age;
  private final double score;
  protected final String[] tags;
  
  @java.beans.ConstructorProperties({"name", "age", "score", "tags"})
  public ValueExample(String name, int age, double score, String[] tags) {
    this.name = name;
    this.age = age;
    this.score = score;
    this.tags = tags;
  }
  
  public String getName() {
    return this.name;
  }
  
  public int getAge() {
    return this.age;
  }
  
  public double getScore() {
    return this.score;
  }
  
  public String[] getTags() {
    return this.tags;
  }
  
  @java.lang.Override
  public boolean equals(Object o) {
    if (o == this) return true;
    if (!(o instanceof ValueExample)) return false;
    final ValueExample other = (ValueExample)o;
    final Object this$name = this.getName();
    final Object other$name = other.getName();
    if (this$name == null ? other$name != null : !this$name.equals(other$name)) return false;
    if (this.getAge() != other.getAge()) return false;
    if (Double.compare(this.getScore(), other.getScore()) != 0) return false;
    if (!Arrays.deepEquals(this.getTags(), other.getTags())) return false;
    return true;
  }
  
  @java.lang.Override
  public int hashCode() {
    final int PRIME = 59;
    int result = 1;
    final Object $name = this.getName();
    result = result * PRIME + ($name == null ? 43 : $name.hashCode());
    result = result * PRIME + this.getAge();
    final long $score = Double.doubleToLongBits(this.getScore());
    result = result * PRIME + (int)($score >>> 32 ^ $score);
    result = result * PRIME + Arrays.deepHashCode(this.getTags());
    return result;
  }
  
  @java.lang.Override
  public String toString() {
    return "ValueExample(name=" + getName() + ", age=" + getAge() + ", score=" + getScore() + ", tags=" + Arrays.deepToString(getTags()) + ")";
  }
  
  ValueExample withAge(int age) {
    return this.age == age ? this : new ValueExample(name, age, score, tags);
  }
  
  public static final class Exercise<T> {
    private final String name;
    private final T value;
    
    private Exercise(String name, T value) {
      this.name = name;
      this.value = value;
    }
    
    public static <T> Exercise<T> of(String name, T value) {
      return new Exercise<T>(name, value);
    }
    
    public String getName() {
      return this.name;
    }
    
    public T getValue() {
      return this.value;
    }
    
    @java.lang.Override
    public boolean equals(Object o) {
      if (o == this) return true;
      if (!(o instanceof ValueExample.Exercise)) return false;
      final Exercise<?> other = (Exercise<?>)o;
      final Object this$name = this.getName();
      final Object other$name = other.getName();
      if (this$name == null ? other$name != null : !this$name.equals(other$name)) return false;
      final Object this$value = this.getValue();
      final Object other$value = other.getValue();
      if (this$value == null ? other$value != null : !this$value.equals(other$value)) return false;
      return true;
    }
    
    @java.lang.Override
    public int hashCode() {
      final int PRIME = 59;
      int result = 1;
      final Object $name = this.getName();
      result = result * PRIME + ($name == null ? 43 : $name.hashCode());
      final Object $value = this.getValue();
      result = result * PRIME + ($value == null ? 43 : $value.hashCode());
      return result;
    }
    
    @java.lang.Override
    public String toString() {
      return "ValueExample.Exercise(name=" + getName() + ", value=" + getValue() + ")";
    }
  }
}

9. @Builder

builder是现在比较推崇的一种构建值对象的方式。

创建一个静态内部类, 使用该类可以使用链式调用创建对象
如 User 对象中存在 name,age 字段, User user=User.builder().name("姓名").age(20).build()

@Builder
public class BuilderExample {
  private String name;
  private int age;
  @Singular private Set<String> occupations;
}

编译后

public class BuilderExample {
  private String name;
  private int age;
  private Set<String> occupations;
  
  BuilderExample(String name, int age, Set<String> occupations) {
    this.name = name;
    this.age = age;
    this.occupations = occupations;
  }
  
  public static BuilderExampleBuilder builder() {
    return new BuilderExampleBuilder();
  }
  
  public static class BuilderExampleBuilder {
    private String name;
    private int age;
    private java.util.ArrayList<String> occupations;
    
    BuilderExampleBuilder() {
    }
    
    public BuilderExampleBuilder name(String name) {
      this.name = name;
      return this;
    }
    
    public BuilderExampleBuilder age(int age) {
      this.age = age;
      return this;
    }
    
    public BuilderExampleBuilder occupation(String occupation) {
      if (this.occupations == null) {
        this.occupations = new java.util.ArrayList<String>();
      }
      
      this.occupations.add(occupation);
      return this;
    }
    
    public BuilderExampleBuilder occupations(Collection<? extends String> occupations) {
      if (this.occupations == null) {
        this.occupations = new java.util.ArrayList<String>();
      }

      this.occupations.addAll(occupations);
      return this;
    }
    
    public BuilderExampleBuilder clearOccupations() {
      if (this.occupations != null) {
        this.occupations.clear();
      }
      
      return this;
    }

    public BuilderExample build() {
      // complicated switch statement to produce a compact properly sized immutable set omitted.
      // go to https://projectlombok.org/features/Singular-snippet.html to see it.
      Set<String> occupations = ...;
      return new BuilderExample(name, age, occupations);
    }
    
    @java.lang.Override
    public String toString() {
      return "BuilderExample.BuilderExampleBuilder(name = " + this.name + ", age = " + this.age + ", occupations = " + this.occupations + ")";
    }
  }
}

10. @SneakyThrows

对标注的方法进行 try catch 后抛出异常, 可在 value 输入需要 catch 的异常数组, 默认 catch Throwable

public class SneakyThrowsExample implements Runnable {
  @SneakyThrows(UnsupportedEncodingException.class)
  public String utf8ToString(byte[] bytes) {
    return new String(bytes, "UTF-8");
  }
  
  @SneakyThrows
  public void run() {
    throw new Throwable();
  }
}

编译后

public class SneakyThrowsExample implements Runnable {
  public String utf8ToString(byte[] bytes) {
    try {
      return new String(bytes, "UTF-8");
    } catch (UnsupportedEncodingException e) {
      throw Lombok.sneakyThrow(e);
    }
  }
  
  public void run() {
    try {
      throw new Throwable();
    } catch (Throwable t) {
      throw Lombok.sneakyThrow(t);
    }
  }
}

11. @Synchronized

在标注的方法内 使用 synchronized($lock) {} 对代码进行包裹 ,$lock 为 new Object[0]

public class SynchronizedExample {
  private final Object readLock = new Object();
  
  @Synchronized
  public static void hello() {
    System.out.println("world");
  }
  
  @Synchronized
  public int answerToLife() {
    return 42;
  }
  
  @Synchronized("readLock")
  public void foo() {
    System.out.println("bar");
  }
}

编译后

public class SynchronizedExample {
  private static final Object $LOCK = new Object[0];
  private final Object $lock = new Object[0];
  private final Object readLock = new Object();
  
  public static void hello() {
    synchronized($LOCK) {
      System.out.println("world");
    }
  }
  
  public int answerToLife() {
    synchronized($lock) {
      return 42;
    }
  }
  
  public void foo() {
    synchronized(readLock) {
      System.out.println("bar");
    }
  }
}

12. @Getter(lazy=true)

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;
  }
}

编译后

public class GetterLazyExample {
  private final java.util.concurrent.AtomicReference<java.lang.Object> cached = new java.util.concurrent.AtomicReference<java.lang.Object>();
  
  public double[] getCached() {
    java.lang.Object value = this.cached.get();
    if (value == null) {
      synchronized(this.cached) {
        value = this.cached.get();
        if (value == null) {
          final double[] actualValue = expensive();
          value = actualValue == null ? this.cached : actualValue;
          this.cached.set(value);
        }
      }
    }
    return (double[])(value == this.cached ? null : value);
  }
  
  private double[] expensive() {
    double[] result = new double[1000000];
    for (int i = 0; i < result.length; i++) {
      result[i] = Math.asin(i);
    }
    return result;
  }
}

13. @Log

@Log
public class LogExample {
  
  public static void main(String... args) {
    log.error("Something's wrong here");
  }
}

@Slf4j
public class LogExampleOther {
  
  public static void main(String... args) {
    log.error("Something else is wrong here");
  }
}

@CommonsLog(topic="CounterLog")
public class LogExampleCategory {

  public static void main(String... args) {
    log.error("Calling the 'CounterLog' with a message");
  }
}

编译后

public class LogExample {
  private static final java.util.logging.Logger log = java.util.logging.Logger.getLogger(LogExample.class.getName());
  
  public static void main(String... args) {
    log.error("Something's wrong here");
  }
}

public class LogExampleOther {
  private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(LogExampleOther.class);
  
  public static void main(String... args) {
    log.error("Something else is wrong here");
  }
}

public class LogExampleCategory {
  private static final org.apache.commons.logging.Log log = org.apache.commons.logging.LogFactory.getLog("CounterLog");

  public static void main(String... args) {
    log.error("Calling the 'CounterLog' with a message");
  }
}

Lombok原理

说道 Lombok，我们就得去提到 JSR 269: Pluggable Annotation Processing API (https://www.jcp.org/en/jsr/detail?id=269) 。JSR 269 之前我们也有注解这样的神器，可是我们比如想要做什么必须使用反射，反射的方法局限性较大。首先，它必须定义@Retention为RetentionPolicy.RUNTIME，只能在运行时通过反射来获取注解值，使得运行时代码效率降低。其次，如果想在编译阶段利用注解来进行一些检查，对用户的某些不合理代码给出错误报告，反射的使用方法就无能为力了。而 JSR 269 之后我们可以在 Javac的编译期利用注解做这些事情。所以我们发现核心的区分是在 运行期 还是 编译期。

从上图可知，Annotation Processing 是在解析和生成之间的一个步骤。

上图是 Lombok 处理流程，在Javac 解析成抽象语法树之后(AST), Lombok 根据自己的注解处理器，动态的修改 AST，增加新的节点(所谓代码)，最终通过分析和生成字节码。