Lombok：讓JAVA代碼更優(yōu)雅
關(guān)于Lombok，其實(shí)在網(wǎng)上可以找到很多如何使用的文章认境，但是很少能找到比較齊全的整理芍瑞。我也一直尋思著想寫一篇各個(gè)注解用法的總結(jié)，但是一直都沒有付諸行動(dòng)藕帜。今天看到了微信公眾號(hào)”原力注入”推送的這篇文章，總結(jié)的內(nèi)容很全惜傲，所以分享給所有關(guān)注我博客的朋友們洽故。
背景
我們?cè)陂_發(fā)過程中，通常都會(huì)定義大量的JavaBean盗誊，然后通過IDE去生成其屬性的構(gòu)造器收津、getter、setter浊伙、equals撞秋、hashcode、toString方法嚣鄙，當(dāng)要對(duì)某個(gè)屬性進(jìn)行改變時(shí)吻贿，比如命名、類型等哑子，都需要重新去生成上面提到的這些方法舅列，那Java中有沒有一種方式能夠避免這種重復(fù)的勞動(dòng)呢？答案是有卧蜓，我們來(lái)看一下下面這張圖帐要，右面是一個(gè)簡(jiǎn)單的JavaBean，只定義了兩個(gè)屬性弥奸，在類上加上了@Data榨惠，從左面的結(jié)構(gòu)圖上可以看到，已經(jīng)自動(dòng)生成了上面提到的方法。

圖片.png

Lombok簡(jiǎn)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上項(xiàng)目介紹所言赠橙，Lombok項(xiàng)目通過添加“處理程序”耽装，使java成為一種更為簡(jiǎn)單的語(yǔ)言∑诰荆可以理解為掉奄，?Lombok是一個(gè)可以通過簡(jiǎn)單的注解形式來(lái)幫助我們簡(jiǎn)化消除一些必須有但顯得很臃腫的Java代碼的工具，通過使用對(duì)應(yīng)的注解凤薛，可以在編譯源碼的時(shí)候生成對(duì)應(yīng)的方法姓建。官方地址：https://projectlombok.org/，github地址：https://github.com/rzwitserloot/lombok缤苫； 作為一個(gè)Old Java Developer,我們都知道我們經(jīng)常需要定義一系列的套路引瀑，比如定義如下的格式對(duì)象。

public class DataExample {
      private final String name;
      private int age;
      private double score;
      private String[] tags;
  }

我們往往需要定義一系列的Get和Set方法最終展示形式如：

 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;
  }
}

那我們有沒有可以簡(jiǎn)化的辦法呢榨馁，第一種就是使用IDEA等IDE提供的一鍵生成的快捷鍵，第二種就是我們今天介紹的 Lombok項(xiàng)目：

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

Wow…這樣就可以完成我們的需求帜矾，簡(jiǎn)直是太棒了翼虫，僅僅需要幾個(gè)注解，我們就擁有了完整的GetSet方法屡萤，還包含了ToString等方法的生成珍剑。

圖片.png

Lombok安裝
整個(gè)Lombok只有一個(gè)Jar包，可到這里下載：https://projectlombok.org/download
Lombok支持多種使用安裝方式死陆，這里我們講最常見的對(duì)兩大IDE的支持：
Eclipse （含延伸版本）
雙擊打開 lombok.jar （前提：你得裝了JDK）, 可見如下頁(yè)面點(diǎn)擊 Install/Update：

圖片.png

恭喜你招拙，已經(jīng)安裝成功了。我們打開 Eclipse 的 About 頁(yè)面我們可以看見措译。

圖片.png

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

翻譯成 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：也就是類型推導(dǎo)啦。
@NonNull
Null 即是罪惡

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

翻譯成 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
自動(dòng)化才是生產(chǎn)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);
    }
  }
}

翻譯成 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里面就已經(jīng)提供 try with resource
@Getter/@Setter
可以作用在類上和屬性上最疆，放在類上杯巨，會(huì)對(duì)所有的非靜態(tài)(non-static)屬性生成Getter/Setter方法，放在屬性上努酸，會(huì)對(duì)該屬性生成Getter/Setter方法服爷。并可以指定Getter/Setter方法的訪問級(jí)別。
再也不寫 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);
  }
}

翻譯成 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，默認(rèn)情況下层扶，會(huì)輸出類名箫章、所有屬性，屬性會(huì)按照順序輸出镜会，以逗號(hào)分割檬寂。
Debug Log 最強(qiáng)幫手

@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;
    }
  }
}

翻譯后：

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：其實(shí)和 org.apache.commons.lang3.builder.ReflectionToStringBuilder 很像。
@NoArgsConstructor, @RequiredArgsConstructor and @AllArgsConstructor
?無(wú)參構(gòu)造器戳表、部分參數(shù)構(gòu)造器桶至、全參構(gòu)造器，當(dāng)我們需要重載多個(gè)構(gòu)造器的時(shí)候匾旭，Lombok就無(wú)能為力了镣屹。

@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;
  }
}

翻譯后：

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
這個(gè)就相當(dāng)?shù)暮?jiǎn)單啦，因?yàn)槲覀儼l(fā)現(xiàn)@ToString, @EqualsAndHashCode, @Getter 都很常用价涝，這個(gè)一個(gè)注解就相當(dāng)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;
  }
}

翻譯后：

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() + ")";
    }
  }
}

我們發(fā)現(xiàn)了 @Value 就是 @Data 的不可變版本女蜈。至于不可變有什么好處∩瘢可有參看此篇（https://blogs.msdn.microsoft.com/ericlippert/2007/11/13/immutability-in-c-part-one-kinds-of-immutability/）
@Builder
我的最愛

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

翻譯后：

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是現(xiàn)在比較推崇的一種構(gòu)建值對(duì)象的方式伪窖。
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();
  }
}

翻譯后

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);
    }
  }
}

很好的隱藏了異常，有時(shí)候的確會(huì)有這樣的煩惱居兆，從某種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");
  }
}

翻譯后

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");
    }
  }
}

這個(gè)就比較簡(jiǎn)單直接添加了synchronized關(guān)鍵字就Ok啦覆山。不過現(xiàn)在JDK也比較推薦的是 Lock 對(duì)象，這個(gè)可能用的不是特別多泥栖。
@Getter(lazy=true)
節(jié)約是美德

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;
  }
}

翻譯后：

    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
再也不用寫那些差不多的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");
  }
}

翻譯后：

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
說道 Lombok簇宽，我們就得去提到 JSR 269: Pluggable Annotation Processing API (https://www.jcp.org/en/jsr/detail?id=269) 。JSR 269 之前我們也有注解這樣的神器吧享，可是我們比如想要做什么必須使用反射魏割，反射的方法局限性較大。首先钢颂，它必須定義@Retention為RetentionPolicy.RUNTIME见妒，只能在運(yùn)行時(shí)通過反射來(lái)獲取注解值，使得運(yùn)行時(shí)代碼效率降低甸陌。其次须揣，如果想在編譯階段利用注解來(lái)進(jìn)行一些檢查，對(duì)用戶的某些不合理代碼給出錯(cuò)誤報(bào)告钱豁，反射的使用方法就無(wú)能為力了耻卡。而 JSR 269 之后我們可以在 Javac的編譯期利用注解做這些事情。所以我們發(fā)現(xiàn)核心的區(qū)分是在 運(yùn)行期 還是 編譯期牲尺。

圖片.png

從上圖可知卵酪，Annotation Processing 是在解析和生成之間的一個(gè)步驟幌蚊。

圖片.png

上圖是 Lombok 處理流程，在Javac 解析成抽象語(yǔ)法樹之后(AST), Lombok 根據(jù)自己的注解處理器溃卡，動(dòng)態(tài)的修改 AST溢豆，增加新的節(jié)點(diǎn)(所謂代碼)，最終通過分析和生成字節(jié)碼瘸羡。
關(guān)于原理我們大致上的描述下漩仙，如果有興趣可以參考下方文檔。
jdk-compilation-overview
Project Lombok: Creating Custom Transformations
Lombok問題
無(wú)法支持多種參數(shù)構(gòu)造器的重載犹赖。

參考：

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

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