十分钟搞懂Lombok使用与原理
1 简介Lombok是一款好用顺手的工具，就像Google Guava一样，在此予以强烈推荐，每一个Java工程师都应该使用它。Lombok是一种Java?实用工具，可用来帮助开发人员消除Java的冗长代码，尤其是对于简单的Java对象（POJO）。它通过注释实现这一目的。通过在开发环境中实现Lombok，开发人员可以节省构建诸如hashCode()和equals()这样的方法以及以往用来分类各种accessor和mutator的大量时间。 2 IntelliJ安装Lombok
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.16.8</version>
</dependency>
3 Lombok用法3.1 Lombok注解说明
3.2 Lombok代码示例
public static void main(String[] args) {
val sets = new HashSet<String>();
val lists = new ArrayList<String>();
val maps = new HashMap<String,String>();
//=>相当于如下
final Set<String> sets2 = new HashSet<>();
final List<String> lists2 = new ArrayList<>();
final Map<String,String> maps2 = new HashMap<>();
}
public void notNullExample(@NonNull String string) {
string.length();
}
//=>相当于
public void notNullExample(String string) {
if (string != null) {
string.length();
} else {
throw new NullPointerException("null");
}
}
public static void main(String[] args) {
try {
@Cleanup InputStream inputStream = new FileInputStream(args[0]);
} catch (FileNotFoundException e) {
e.printStackTrace();
}
//=>相当于
InputStream inputStream = null;
try {
inputStream = new FileInputStream(args[0]);
} catch (FileNotFoundException e) {
e.printStackTrace();
} finally {
if (inputStream != null) {
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
@Setter(AccessLevel.PUBLIC) @Getter(AccessLevel.PROTECTED) private int id; private String shap;
@ToString(exclude = "id",callSuper = true,includeFieldNames = true)
public class LombokDemo {
private int id;
private String name;
private int age;
public static void main(String[] args) {
//输出LombokDemo([email?protected],name=null,age=0)
System.out.println(new LombokDemo());
}
}
@EqualsAndHashCode(exclude = {"id","shape"},callSuper = false)
public class LombokDemo {
private int id;
private String shap;
}
@NoArgsConstructor
@RequiredArgsConstructor(staticName = "of")
@AllArgsConstructor
public class LombokDemo {
@NonNull
private int id;
@NonNull
private String shap;
private int age;
public static void main(String[] args) {
new LombokDemo(1,"circle");
//使用静态工厂方法
LombokDemo.of(2,"circle");
//无参构造
new LombokDemo();
//包含所有参数
new LombokDemo(1,"circle",2);
}
}
import lombok.Data;
@Data
public class Menu {
private String shopId;
private String skuMenuId;
private String skuName;
private String normalizeSkuName;
private String dishMenuId;
private String dishName;
private String dishNum;
//默认阈值
private float thresHold = 0;
//新阈值
private float newThresHold = 0;
//总得分
private float totalScore = 0;
}
@Value
public class LombokDemo {
@NonNull
private int id;
@NonNull
private String shap;
private int age;
//相当于
private final int id;
public int getId() {
return this.id;
}
...
}
@Builder
public class BuilderExample {
private String name;
private int age;
@Singular
private Set<String> occupations;
public static void main(String[] args) {
BuilderExample test = BuilderExample.builder().age(11).name("test").build();
}
}
import lombok.SneakyThrows;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;
public class Test {
@SneakyThrows()
public void read() {
InputStream inputStream = new FileInputStream("");
}
@SneakyThrows
public void write() {
throw new UnsupportedEncodingException();
}
//相当于
public void read() throws FileNotFoundException {
InputStream inputStream = new FileInputStream("");
}
public void write() throws UnsupportedEncodingException {
throw new UnsupportedEncodingException();
}
}
public class SynchronizedDemo {
@Synchronized
public static void hello() {
System.out.println("world");
}
//相当于
private static final Object $LOCK = new Object[0];
public static void hello() {
synchronized ($LOCK) {
System.out.println("world");
}
}
}
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;
}
}
// 相当于如下所示:
import java.util.concurrent.atomic.AtomicReference;
public class GetterLazyExample {
private final AtomicReference<java.lang.Object> cached = new AtomicReference<>();
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;
}
}
4 Lombok注解原理说道 Lombok，我们就得去提到 JSR 269: Pluggable Annotation Processing API (www.jcp.org/en/jsr/deta…) 。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。具体详细步骤如下：
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。 自从Java 6起，javac就支持“JSR 269 Pluggable Annotation Processing API”规范，只要程序实现了该API，就能在javac运行的时候得到调用。
IDE工具问题解决：
5 自定义支持JSR269的注解一般javac的编译过程，java文件首先通过进行解析构建出一个AST，然后执行注解处理，最后经过分析优化生成二进制的.class文件。我们能做到的是，在注解处理阶段进行一些相应处理。首先我们在META-INF.services下创建如下文件：
文件中指定我们的注解处理器：com.alipay.kris.other.lombok.MyAnnotaionProcessor，然后我们接可以编写自己的注解处理器，一个简单的实例代码如下： @SupportedSourceVersion(SourceVersion.RELEASE_8)
@SupportedAnnotationTypes("com.alipay.kris.other.lombok.*")
public class MyAnnotaionProcessor extends AbstractProcessor {
public MyAnnotaionProcessor() {
super();
}
@Override
public boolean process(Set<? extends TypeElement> annotations,RoundEnvironment roundEnv) {
for (Element elem : roundEnv.getElementsAnnotatedWith(MyAnnotation.class)) {
MyAnnotation annotation = elem.getAnnotation(MyAnnotation.class);
String message = "annotation found in " + elem.getSimpleName()
+ " with " + annotation.value();
addToString(elem);
processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE,message);
}
return true; // no further processing of this annotation type
}
}
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