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，但是很少能找到比較齊全的整理。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)。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，所以分享給所有關(guān)注我博客的朋友們。
背景
我們?cè)陂_發(fā)過程中，通常都會(huì)定義大量的JavaBean，然后通過IDE去生成其屬性的構(gòu)造器、getter、setter、equals、hashcode、toString方法，當(dāng)要對(duì)某個(gè)屬性進(jìn)行改變時(shí)，比如命名、類型等，都需要重新去生成上面提到的這些方法，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，我們來看一下下面這張圖，右面是一個(gè)簡單的JavaBean，只定義了兩個(gè)屬性，在類上加上了@Data，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。
Lombok簡介
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，Lombok項(xiàng)目通過添加“處理程序”，使java成為一種更為簡單的語言。可以理解為，?Lombok是一個(gè)可以通過簡單的注解形式來幫助我們簡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，可以在編譯源碼的時(shí)候生成對(duì)應(yīng)的方法。官方地址：https://projectlombok.org/，github地址：https://github.com/rzwitserloot/lombok； 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，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化的辦法呢，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，簡直是太棒了，僅僅需要幾個(gè)注解，我們就擁有了完整的GetSet方法，還包含了ToString等方法的生成。
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，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）, 可見如下頁面點(diǎn)擊 Install/Update：
恭喜你，已經(jīng)安裝成功了。我們打開 Eclipse 的 About 頁面我們可以看見。
IntelliJ IDEA
定位到 File > Settings > Plugins
點(diǎn)擊 Browse repositories…
搜索 Lombok Plugin
點(diǎn)擊 Install plugin
重啟 IDEA
更多安裝請(qǐng)參考：https://projectlombok.org/
Lombok使用
Lombok 其實(shí)也不能算是一個(gè)特別新的項(xiàng)目，從 2011 開始在中心倉庫提供支持，現(xiàn)在也分為 stable 和 experimental 兩個(gè)版本，本文側(cè)重介紹 stable 功能：
val
如果對(duì)其他的語言有研究的會(huì)發(fā)現(xiàn)，很多語言是使用 var 作為變量申明，val作為常量申明。這里的val也是這個(gè)作用。
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，放在類上，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，會(huì)對(duì)該屬性生成Getter/Setter方法。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，會(huì)輸出類名、所有屬性，屬性會(huì)按照順序輸出，以逗號(hào)分割。
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
?無參構(gòu)造器、部分參數(shù)構(gòu)造器、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，Lombok就無能為力了。
@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)暮唵卫玻驗(yàn)槲覀儼l(fā)現(xiàn)@ToString, @EqualsAndHashCode, @Getter 都很常用，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。至于不可變有什么好處?？捎袇⒖创似?a target="_blank">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。
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，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)就比較簡單直接添加了synchronized關(guān)鍵字就Ok啦。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。
@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，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，可是我們比如想要做什么必須使用反射，反射的方法局限性較大。首先，它必須定義@Retention為RetentionPolicy.RUNTIME，只能在運(yùn)行時(shí)通過反射來獲取注解值，使得運(yùn)行時(shí)代碼效率降低。其次，如果想在編譯階段利用注解來進(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，反射的使用方法就無能為力了。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。
從上圖可知，Annotation Processing 是在解析和生成之間的一個(gè)步驟。
上圖是 Lombok 處理流程，在Javac 解析成抽象語法樹之后(AST), Lombok 根據(jù)自己的注解處理器，動(dòng)態(tài)的修改 AST，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。
關(guān)于原理我們大致上的描述下，如果有興趣可以參考下方文檔。
jdk-compilation-overview
Project Lombok: Creating Custom Transformations
Lombok問題
無法支持多種參數(shù)構(gòu)造器的重載。
參考：
