效率提高 10 倍！一份不可多得的 Lombok 學習指南

2019 年 12 月 2 日
筆記

一、Lombok 簡介

Lombok 是一款 Java 開發插件，使得 Java 開發者可以通過其定義的一些註解來消除業務工程中冗長和繁瑣的程式碼，尤其對於簡單的 Java 模型對象（POJO）。在開發環境中使用 Lombok 插件後，Java 開發人員可以節省出重複構建，諸如 hashCode 和 equals 這樣的方法以及各種業務對象模型的 accessor 和 toString 等方法的大量時間。對於這些方法，Lombok 能夠在編譯源程式碼期間自動幫我們生成這些方法，但並不會像反射那樣降低程式的性能。

二、Lombok 安裝

2.1 構建工具

Gradle

在 build.gradle 文件中添加 lombok 依賴：

dependencies {      compileOnly 'org.projectlombok:lombok:1.18.10'      annotationProcessor 'org.projectlombok:lombok:1.18.10'  }

Maven

在 Maven 項目的 pom.xml 文件中添加 lombok 依賴：

<dependency>          <groupId>org.projectlombok</groupId>          <artifactId>lombok</artifactId>          <version>1.18.10</version>          <scope>provided</scope>  </dependency>

Ant

假設在 lib 目錄中已經存在 lombok.jar，然後設置 javac 任務：

<javac srcdir="src" destdir="build" source="1.8">      <classpath location="lib/lombok.jar" />  </javac>

2.2 IDE

由於 Lombok 僅在編譯階段生成程式碼，所以使用 Lombok 註解的源程式碼，在 IDE 中會被高亮顯示錯誤，針對這個問題可以通過安裝 IDE 對應的插件來解決。這裡不詳細展開，具體的安裝方式可以參考 Setting up Lombok with Eclipse and IntelliJ 這篇文章。

三、Lombok 詳解

注意：以下示例所使用的 Lombok 版本是 1.18.10

3.1 @Getter and @Setter 註解

你可以使用 @Getter 或 @Setter 注釋任何類或欄位，Lombok 會自動生成默認的 getter/setter 方法。

@Getter 註解

@Target({ElementType.FIELD, ElementType.TYPE})  @Retention(RetentionPolicy.SOURCE)  public @interface Getter {        lombok.AccessLevel value() default lombok.AccessLevel.PUBLIC;      AnyAnnotation[] onMethod() default {};        boolean lazy() default false;  }

@Setter 註解

@Target({ElementType.FIELD, ElementType.TYPE})  @Retention(RetentionPolicy.SOURCE)  public @interface Setter {        lombok.AccessLevel value() default lombok.AccessLevel.PUBLIC;      AnyAnnotation[] onMethod() default {};      AnyAnnotation[] onParam() default {};  }

使用示例

package com.semlinker.lombok;    @Getter  @Setter  public class GetterAndSetterDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class GetterAndSetterDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;        public GetterAndSetterDemo() {      }          public String getFirstName() {          return this.firstName;      }        public void setFirstName(String firstName) {          this.firstName = firstName;      }  }

Lazy Getter

@Getter 註解支援一個 lazy 屬性，該屬性默認為 false。當設置為 true 時，會啟用延遲初始化，即當首次調用 getter 方法時才進行初始化。

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示例

package com.semlinker.lombok;    public class LazyGetterDemo {      public static void main(String[] args) {          LazyGetterDemo m = new LazyGetterDemo();          System.out.println("Main instance is created");          m.getLazy();      }        @Getter      private final String notLazy = createValue("not lazy");        @Getter(lazy = true)      private final String lazy = createValue("lazy");        private String createValue(String name) {          System.out.println("createValue(" + name + ")");          return null;      }  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class LazyGetterDemo {      private final String notLazy = this.createValue("not lazy");      private final AtomicReference<Object> lazy = new AtomicReference();          public String getNotLazy() {          return this.notLazy;      }        public String getLazy() {          Object value = this.lazy.get();          if (value == null) {              synchronized(this.lazy) {                  value = this.lazy.get();                  if (value == null) {                      String actualValue = this.createValue("lazy");                      value = actualValue == null ? this.lazy : actualValue;                      this.lazy.set(value);                  }              }          }            return (String)((String)(value == this.lazy ? null : value));      }  }

通過以上程式碼可知，調用 getLazy 方法時，若發現 value 為 null，則會在同步程式碼塊中執行初始化操作。

3.2 Constructor Annotations

@NoArgsConstructor 註解

使用 @NoArgsConstructor 註解可以為指定類，生成默認的構造函數，@NoArgsConstructor 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface NoArgsConstructor {        String staticName() default "";      AnyAnnotation[] onConstructor() default {};        AccessLevel access() default lombok.AccessLevel.PUBLIC;        boolean force() default false;  }

示例

package com.semlinker.lombok;    @NoArgsConstructor(staticName = "getInstance")  public class NoArgsConstructorDemo {      private long id;      private String name;      private int age;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class NoArgsConstructorDemo {      private long id;      private String name;      private int age;        private NoArgsConstructorDemo() {      }        public static NoArgsConstructorDemo getInstance() {          return new NoArgsConstructorDemo();      }  }

@AllArgsConstructor 註解

使用 @AllArgsConstructor 註解可以為指定類，生成包含所有成員的構造函數，@AllArgsConstructor 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface AllArgsConstructor {        String staticName() default "";      AnyAnnotation[] onConstructor() default {};        AccessLevel access() default lombok.AccessLevel.PUBLIC;  }

示例

package com.semlinker.lombok;    @AllArgsConstructor  public class AllArgsConstructorDemo {      private long id;      private String name;      private int age;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class AllArgsConstructorDemo {      private long id;      private String name;      private int age;        public AllArgsConstructorDemo(long id, String name, int age) {          this.id = id;          this.name = name;          this.age = age;      }  }

@RequiredArgsConstructorDemo 註解

使用 @RequiredArgsConstructor 註解可以為指定類必需初始化的成員變數，如 final 成員變數，生成對應的構造函數，@RequiredArgsConstructor 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface RequiredArgsConstructor {        String staticName() default "";      AnyAnnotation[] onConstructor() default {};        AccessLevel access() default lombok.AccessLevel.PUBLIC;  }

示例

package com.semlinker.lombok;    @RequiredArgsConstructor  public class RequiredArgsConstructorDemo {      private final long id;      private String name;      private int age;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class RequiredArgsConstructorDemo {      private final long id;      private String name;      private int age;        public RequiredArgsConstructorDemo(long id) {          this.id = id;      }  }

3.3 @EqualsAndHashCode 註解

使用 @EqualsAndHashCode 註解可以為指定類生成 equals 和 hashCode 方法， @EqualsAndHashCode 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface EqualsAndHashCode {        String[] exclude() default {};        String[] of() default {};        boolean callSuper() default false;        boolean doNotUseGetters() default false;        AnyAnnotation[] onParam() default {};        @Deprecated      @Retention(RetentionPolicy.SOURCE)      @Target({})      @interface AnyAnnotation {}        @Target(ElementType.FIELD)      @Retention(RetentionPolicy.SOURCE)      public @interface Exclude {}        @Target({ElementType.FIELD, ElementType.METHOD})      @Retention(RetentionPolicy.SOURCE)      public @interface Include {          String replaces() default "";      }  }

示例

package com.semlinker.lombok;    @EqualsAndHashCode  public class EqualsAndHashCodeDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class EqualsAndHashCodeDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;        public EqualsAndHashCodeDemo() {      }        public boolean equals(Object o) {          if (o == this) {              return true;          } else if (!(o instanceof EqualsAndHashCodeDemo)) {              return false;          } else {              EqualsAndHashCodeDemo other = (EqualsAndHashCodeDemo)o;              if (!other.canEqual(this)) {                  return false;              } else {            }      }        public int hashCode() {          int PRIME = true;          int result = 1;          Object $firstName = this.firstName;          int result = result * 59 + ($firstName == null ? 43 : $firstName.hashCode());          Object $lastName = this.lastName;          result = result * 59 + ($lastName == null ? 43 : $lastName.hashCode());          Object $dateOfBirth = this.dateOfBirth;          result = result * 59 + ($dateOfBirth == null ? 43 : $dateOfBirth.hashCode());          return result;      }  }

3.4 @ToString 註解

使用 @ToString 註解可以為指定類生成 toString 方法， @ToString 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface ToString {        boolean includeFieldNames() default true;        String[] exclude() default {};        String[] of() default {};          boolean callSuper() default false;        boolean doNotUseGetters() default false;        boolean onlyExplicitlyIncluded() default false;        @Target(ElementType.FIELD)      @Retention(RetentionPolicy.SOURCE)      public @interface Exclude {}        @Target({ElementType.FIELD, ElementType.METHOD})      @Retention(RetentionPolicy.SOURCE)      public @interface Include {          int rank() default 0;          String name() default "";      }  }

示例

package com.semlinker.lombok;    @ToString(exclude = {"dateOfBirth"})  public class ToStringDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class ToStringDemo {      String firstName;      String lastName;      LocalDate dateOfBirth;        public ToStringDemo() {      }        public String toString() {          return "ToStringDemo(firstName=" + this.firstName + ", lastName=" +            this.lastName + ")";      }  }

3.5 @Data 註解

@Data 註解與同時使用以下的註解的效果是一樣的：

@ToString
@Getter
@Setter
@RequiredArgsConstructor
@EqualsAndHashCode

@Data 註解的定義如下：

@Target(ElementType.TYPE)  @Retention(RetentionPolicy.SOURCE)  public @interface Data {      String staticConstructor() default "";  }

示例

package com.semlinker.lombok;    @Data  public class DataDemo {      private Long id;      private String summary;      private String description;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class DataDemo {      private Long id;      private String summary;      private String description;        public DataDemo() {      }          public Long getId() {          return this.id;      }        public void setId(Long id) {          this.id = id;      }        public boolean equals(Object o) {          if (o == this) {              return true;          } else if (!(o instanceof DataDemo)) {              return false;          } else {              DataDemo other = (DataDemo)o;              if (!other.canEqual(this)) {                  return false;              } else {                }          }      }        protected boolean canEqual(Object other) {          return other instanceof DataDemo;      }        public int hashCode() {          int PRIME = true;          int result = 1;          Object $id = this.getId();          int result = result * 59 + ($id == null ? 43 : $id.hashCode());          Object $summary = this.getSummary();          result = result * 59 + ($summary == null ? 43 : $summary.hashCode());          Object $description = this.getDescription();          result = result * 59 + ($description == null ? 43 : $description.hashCode());          return result;      }        public String toString() {          return "DataDemo(id=" + this.getId() + ", summary=" + this.getSummary() + ", description=" + this.getDescription() + ")";      }  }

3.6 @Log 註解

若你將 @Log 的變體放在類上（適用於你所使用的日誌記錄系統的任何一種）；之後，你將擁有一個靜態的 final log 欄位，然後你就可以使用該欄位來輸出日誌。

@Log  private static final java.util.logging.Logger log = java.util.logging.Logger.getLogger(LogExample.class.getName());    @Log4j  private static final org.apache.log4j.Logger log = org.apache.log4j.Logger.getLogger(LogExample.class);    @Log4j2  private static final org.apache.logging.log4j.Logger log = org.apache.logging.log4j.LogManager.getLogger(LogExample.class);    @Slf4j  private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(LogExample.class);    @XSlf4j  private static final org.slf4j.ext.XLogger log = org.slf4j.ext.XLoggerFactory.getXLogger(LogExample.class);    @CommonsLog  private static final org.apache.commons.logging.Log log = org.apache.commons.logging.LogFactory.getLog(LogExample.class);

3.7 @Synchronized 註解

@Synchronized 是同步方法修飾符的更安全的變體。與 synchronized 一樣，該註解只能應用在靜態和實例方法上。它的操作類似於 synchronized 關鍵字，但是它鎖定在不同的對象上。synchronized 關鍵字應用在實例方法時，鎖定的是 this 對象，而應用在靜態方法上鎖定的是類對象。對於 @Synchronized 註解聲明的方法來說，它鎖定的是 $LOCK 或 $lock。@Synchronized 註解的定義如下：

@Target(ElementType.METHOD)  @Retention(RetentionPolicy.SOURCE)  public @interface Synchronized {        String value() default "";  }

示例

package com.semlinker.lombok;    public class SynchronizedDemo {      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");      }  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

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

3.8 @Builder 註解

使用 @Builder 註解可以為指定類實現建造者模式，該註解可以放在類、構造函數或方法上。@Builder 註解的定義如下：

@Target({TYPE, METHOD, CONSTRUCTOR})  @Retention(SOURCE)  public @interface Builder {      @Target(FIELD)      @Retention(SOURCE)      public @interface Default {}          String builderMethodName() default "builder";        String buildMethodName() default "build";        String builderClassName() default "";        boolean toBuilder() default false;        AccessLevel access() default lombok.AccessLevel.PUBLIC;        @Target({FIELD, PARAMETER})      @Retention(SOURCE)      public @interface ObtainVia {          String field() default "";          String method() default "";          boolean isStatic() default false;      }  }

示例

package com.semlinker.lombok;    @Builder  public class BuilderDemo {      private final String firstname;      private final String lastname;      private final String email;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class BuilderDemo {      private final String firstname;      private final String lastname;      private final String email;        BuilderDemo(String firstname, String lastname, String email) {          this.firstname = firstname;          this.lastname = lastname;          this.email = email;      }        public static BuilderDemo.BuilderDemoBuilder builder() {          return new BuilderDemo.BuilderDemoBuilder();      }        public static class BuilderDemoBuilder {          private String firstname;          private String lastname;          private String email;            BuilderDemoBuilder() {          }            public BuilderDemo.BuilderDemoBuilder firstname(String firstname) {              this.firstname = firstname;              return this;          }            public BuilderDemo.BuilderDemoBuilder lastname(String lastname) {              this.lastname = lastname;              return this;          }            public BuilderDemo.BuilderDemoBuilder email(String email) {              this.email = email;              return this;          }            public BuilderDemo build() {              return new BuilderDemo(this.firstname, this.lastname, this.email);          }            public String toString() {              return "BuilderDemo.BuilderDemoBuilder(firstname=" + this.firstname + ", lastname=" + this.lastname + ", email=" + this.email + ")";          }      }  }

3.9 @SneakyThrows 註解

@SneakyThrows 註解用於自動拋出已檢查的異常，而無需在方法中使用 throw 語句顯式拋出。@SneakyThrows 註解的定義如下：

@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})  @Retention(RetentionPolicy.SOURCE)  public @interface SneakyThrows {        Class<? extends Throwable>[] value() default java.lang.Throwable.class;  }

示例

package com.semlinker.lombok;    public class SneakyThrowsDemo {      @SneakyThrows      @Override      protected Object clone() {          return super.clone();      }  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class SneakyThrowsDemo {      public SneakyThrowsDemo() {      }        protected Object clone() {          try {              return super.clone();          } catch (Throwable var2) {              throw var2;          }      }  }

3.10 @NonNull 註解

你可以在方法或構造函數的參數上使用 @NonNull 註解，它將會為你自動生成非空校驗語句。@NonNull 註解的定義如下：

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE, ElementType.TYPE_USE})  @Retention(RetentionPolicy.CLASS)  @Documented  public @interface NonNull {  }

示例

package com.semlinker.lombok;    public class NonNullDemo {      @Getter      @Setter      @NonNull      private String name;  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class NonNullDemo {      @NonNull      private String name;        public NonNullDemo() {      }        @NonNull      public String getName() {          return this.name;      }        public void setName(@NonNull String name) {          if (name == null) {              throw new NullPointerException("name is marked non-null but is null");          } else {              this.name = name;          }      }  }

3.11 @Clean 註解

@Clean 註解用於自動管理資源，用在局部變數之前，在當前變數範圍內即將執行完畢退出之前會自動清理資源，自動生成 try-finally 這樣的程式碼來關閉流。

@Target(ElementType.LOCAL_VARIABLE)  @Retention(RetentionPolicy.SOURCE)  public @interface Cleanup {        String value() default "close";  }

示例

package com.semlinker.lombok;    public class CleanupDemo {      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);          }      }  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

package com.semlinker.lombok;    public class CleanupDemo {      public CleanupDemo() {      }        public static void main(String[] args) throws IOException {          FileInputStream in = new FileInputStream(args[0]);            try {              FileOutputStream out = new FileOutputStream(args[1]);                try {                  byte[] b = new byte[10000];                    while(true) {                      int r = in.read(b);                      if (r == -1) {                          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();              }          }      }  }

3.11 @With 註解

在類的欄位上應用 @With 註解之後，將會自動生成一個 withFieldName(newValue) 的方法，該方法會基於 newValue 調用相應構造函數，創建一個當前類對應的實例。@With 註解的定義如下：

@Target({ElementType.FIELD, ElementType.TYPE})  @Retention(RetentionPolicy.SOURCE)  public @interface With {      AccessLevel value() default AccessLevel.PUBLIC;        With.AnyAnnotation[] onMethod() default {};        With.AnyAnnotation[] onParam() default {};        @Deprecated      @Retention(RetentionPolicy.SOURCE)      @Target({})      public @interface AnyAnnotation {      }  }

示例

public class WithDemo {      @With(AccessLevel.PROTECTED)      @NonNull      private final String name;      @With      private final int age;        public WithDemo(String name, int age) {          if (name == null) throw new NullPointerException();          this.name = name;          this.age = age;      }  }

以上程式碼經過 Lombok 編譯後，會生成如下程式碼：

public class WithDemo {      @NonNull      private final String name;      private final int age;        public WithDemo(String name, int age) {          if (name == null) {              throw new NullPointerException();          } else {              this.name = name;              this.age = age;          }      }        protected WithDemo withName(@NonNull String name) {          if (name == null) {              throw new NullPointerException("name is marked non-null but is null");          } else {              return this.name == name ? this : new WithDemo(name, this.age);          }      }        public WithDemo withAge(int age) {          return this.age == age ? this : new WithDemo(this.name, age);      }  }

3.12 其它特性

val

val 用在局部變數前面，相當於將變數聲明為 final，此外 Lombok 在編譯時還會自動進行類型推斷。val 的使用示例：

public class ValExample {    public String example() {      val example = new ArrayList<String>();      example.add("Hello, World!");      val foo = example.get(0);      return foo.toLowerCase();    }      public void example2() {      val map = new HashMap<Integer, String>();      map.put(0, "zero");      map.put(5, "five");      for (val entry : map.entrySet()) {        System.out.printf("%d: %sn", entry.getKey(), entry.getValue());      }    }  }

以上程式碼等價於：

public class ValExample {    public String example() {      final ArrayList<String> example = new ArrayList<String>();      example.add("Hello, World!");      final String foo = example.get(0);      return foo.toLowerCase();    }      public void example2() {      final HashMap<Integer, String> map = new HashMap<Integer, String>();      map.put(0, "zero");      map.put(5, "five");      for (final Map.Entry<Integer, String> entry : map.entrySet()) {        System.out.printf("%d: %sn", entry.getKey(), entry.getValue());      }    }  }

四、參考資源

Lombok 官網
lombok-cheatsheet
Java開發神器Lombok的使用與原理

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