Lombok：让JAVA代码更优雅
关于Lombok，其实在网上可以找到很多如何使用的文章，但是很少能找到比较齐全的整理。我也一直寻思着想写一篇各个注解用法的总结，但是一直都没有付诸行动。今天看到了微信公众号”原力注入”推送的这篇文章，总结的内容很全，所以分享给所有关注我博客的朋友们。
背景
我们在开发过程中，通常都会定义大量的JavaBean，然后通过IDE去生成其属性的构造器、getter、setter、equals、hashcode、toString方法，当要对某个属性进行改变时，比如命名、类型等，都需要重新去生成上面提到的这些方法，那Java中有没有一种方式能够避免这种重复的劳动呢？答案是有，我们来看一下下面这张图，右面是一个简单的JavaBean，只定义了两个属性，在类上加上了@Data，从左面的结构图上可以看到，已经自动生成了上面提到的方法。

图片.png

Lombok简介
Project Lombok makes java a spicier language by adding ‘handlers’ that know >how to build and compile simple, boilerplate-free, not-quite-java code.
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。可以理解为， Lombok是一个可以通过简单的注解形式来帮助我们简化消除一些必须有但显得很臃肿的Java代码的工具，通过使用对应的注解，可以在编译源码的时候生成对应的方法。官方地址：https://projectlombok.org/，github地址：https://github.com/rzwitserloot/lombok； 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。

图片.png

Lombok安装
整个Lombok只有一个Jar包，可到这里下载：https://projectlombok.org/download
Lombok支持多种使用安装方式，这里我们讲最常见的对两大IDE的支持：
Eclipse （含延伸版本）
双击打开 lombok.jar （前提：你得装了JDK）, 可见如下页面点击 Install/Update：

图片.png

恭喜你，已经安装成功了。我们打开 Eclipse 的 About 页面我们可以看见。

图片.png

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();
}

作者注：也就是类型推导啦。
@NonNull
Null 即是罪恶

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();
  }
}

@Cleanup
自动化才是生产力

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();
      }
    }
  }
}

作者注： JKD7里面就已经提供 try with resource
@Getter/@Setter
可以作用在类上和属性上，放在类上，会对所有的非静态(non-static)属性生成Getter/Setter方法，放在属性上，会对该属性生成Getter/Setter方法。并可以指定Getter/Setter方法的访问级别。
再也不写 public int getFoo() {return foo;}

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;
  }
}

@ToString
生成toString方法，默认情况下，会输出类名、所有属性，属性会按照顺序输出，以逗号分割。
Debug Log 最强帮手

@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;
    }
  }
}

翻译后：

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) + ")";
  }
}

作者注：其实和 org.apache.commons.lang3.builder.ReflectionToStringBuilder 很像。
@NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor
 无参构造器、部分参数构造器、全参构造器，当我们需要重载多个构造器的时候，Lombok就无能为力了。

@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;
  }
}

翻译后：

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() {
    }
  }
}

@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
@Value

    @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;
  }
}

翻译后：

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() + ")";
    }
  }
}

我们发现了 @Value 就是 @Data 的不可变版本。至于不可变有什么好处。可有参看此篇（https://blogs.msdn.microsoft.com/ericlippert/2007/11/13/immutability-in-c-part-one-kinds-of-immutability/）
@Builder
我的最爱

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

翻译后：

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 + ")";
    }
  }
}

builder是现在比较推崇的一种构建值对象的方式。
作者注：生成器模式
@SneakyThrows
to RuntimeException 小助手

    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();
  }
}

翻译后

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);
    }
  }
}

很好的隐藏了异常，有时候的确会有这样的烦恼，从某种程度上也是遵循的了 let is crash

@Synchronized
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");
  }
}

翻译后

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");
    }
  }
}

这个就比较简单直接添加了synchronized关键字就Ok啦。不过现在JDK也比较推荐的是 Lock 对象，这个可能用的不是特别多。
@Getter(lazy=true)
节约是美德

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;
  }
}

翻译后：

    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;
  }
}

@Log
再也不用写那些差不多的LOG啦

@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");
  }
}

翻译后：

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。

图片.png

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

图片.png

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。
关于原理我们大致上的描述下，如果有兴趣可以参考下方文档。
jdk-compilation-overview
Project Lombok: Creating Custom Transformations
Lombok问题
无法支持多种参数构造器的重载。

参考：

• http://blog.didispace.com/java-lombok-1/
  http://blog.csdn.net/ghsau/article/details/52334762

• https://blog.csdn.net/motui/article/details/79012846