lombok簡介
lombok是一個java庫,致力於通過一組註解消除代碼中的一些必要但是臃腫的樣板代碼,精簡代碼,提高效率,還有耍酷。
如何使用
使用lombok需要在IDE中引入對應的插件,並在項目中引入對應的pom依賴
安裝插件
在IDEA的插件中搜索lombok然後安裝
引入依賴
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.10</version>
</dependency>
註解
lombok提供了一系列的註解來幫助我們簡化代碼,下面我們分別對其中一些高頻的註解怎麼使用進行介紹
@Getter / @Setter
@Getter和@Setter註解可以作用在類上,也可以作用在字段上。
使用前我們的寫法如下:
//code1
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
public Long getPersonId() {
return personId;
}
public void setPersonId(Long personId) {
this.personId = personId;
}
public String getMisNum() {
return misNum;
}
public void setMisNum(String misNum) {
this.misNum = misNum;
}
public String getFullName() {
return fullName;
}
public void setFullName(String fullName) {
this.fullName = fullName;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
使用後我們的寫法如下:
//code2
import lombok.Getter;
import lombok.Setter;
@Getter
@Setter
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
}
查看編譯後的class文件,在編譯後會自動生成對應的get和set方法
//code2.class
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
public Person() {
}
public Long getPersonId() {
return this.personId;
}
public String getMisNum() {
return this.misNum;
}
public String getFullName() {
return this.fullName;
}
public String getName() {
return this.name;
}
public void setPersonId(Long personId) {
this.personId = personId;
}
public void setMisNum(String misNum) {
this.misNum = misNum;
}
public void setFullName(String fullName) {
this.fullName = fullName;
}
public void setName(String name) {
this.name = name;
}
}
@ToString
使用前我們的寫法如下:
//code3
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("{");
sb.append("\"personId\":")
.append(personId);
sb.append(",\"misNum\":\"")
.append(misNum).append('\"');
sb.append(",\"fullName\":\"")
.append(fullName).append('\"');
sb.append(",\"name\":\"")
.append(name).append('\"');
sb.append('}');
return sb.toString();
}
}
使用後我們的寫法如下:
//code4
import lombok.ToString;
@ToString
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
}
編譯後的class文件如下,會自動生成toString()方法,但是比較遺憾的是不夠靈活,沒有辦法直接生成Json格式的toString() 方法
//code4.class
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
public Person() {
}
public String toString() {
return "Person(personId=" + this.personId + ", misNum=" + this.misNum + ", fullName=" + this.fullName + ", name=" + this.name + ")";
}
}
@ToString還提供了一些參數
callSuper = true 可以打印父類,指定exclude可以排除字段,不過都比較雞肋。
@Data
使用@Data寫法如下:
//code5
@Data
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
}
編譯後的class文件如下:
//code5.class
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
public Person() {
}
public Long getPersonId() {
return this.personId;
}
public String getMisNum() {
return this.misNum;
}
public String getFullName() {
return this.fullName;
}
public String getName() {
return this.name;
}
public void setPersonId(Long personId) {
this.personId = personId;
}
public void setMisNum(String misNum) {
this.misNum = misNum;
}
public void setFullName(String fullName) {
this.fullName = fullName;
}
public void setName(String name) {
this.name = name;
}
public boolean equals(Object o) {
if (o == this) {
return true;
} else if (!(o instanceof Person)) {
return false;
} else {
Person other = (Person)o;
if (!other.canEqual(this)) {
return false;
} else {
label59: {
Object this$personId = this.getPersonId();
Object other$personId = other.getPersonId();
if (this$personId == null) {
if (other$personId == null) {
break label59;
}
} else if (this$personId.equals(other$personId)) {
break label59;
}
return false;
}
Object this$misNum = this.getMisNum();
Object other$misNum = other.getMisNum();
if (this$misNum == null) {
if (other$misNum != null) {
return false;
}
} else if (!this$misNum.equals(other$misNum)) {
return false;
}
Object this$fullName = this.getFullName();
Object other$fullName = other.getFullName();
if (this$fullName == null) {
if (other$fullName != null) {
return false;
}
} else if (!this$fullName.equals(other$fullName)) {
return false;
}
Object this$name = this.getName();
Object other$name = other.getName();
if (this$name == null) {
if (other$name != null) {
return false;
}
} else if (!this$name.equals(other$name)) {
return false;
}
return true;
}
}
}
protected boolean canEqual(Object other) {
return other instanceof Person;
}
public int hashCode() {
int PRIME = true;
int result = 1;
Object $personId = this.getPersonId();
int result = result * 59 + ($personId == null ? 43 : $personId.hashCode());
Object $misNum = this.getMisNum();
result = result * 59 + ($misNum == null ? 43 : $misNum.hashCode());
Object $fullName = this.getFullName();
result = result * 59 + ($fullName == null ? 43 : $fullName.hashCode());
Object $name = this.getName();
result = result * 59 + ($name == null ? 43 : $name.hashCode());
return result;
}
public String toString() {
return "Person(personId=" + this.getPersonId() + ", misNum=" + this.getMisNum() + ", fullName=" + this.getFullName() + ", name=" + this.getName() + ")";
}
}
@Data註解自動生成了getter/setter方法、toString()方法、覆寫了hashCode()和equals()方法.
@Slf4j
使用@Slf4j註解可以省去實例化log對象的代碼
使用方式如下:
//code6
@Slf4j
public class Person {
}
編譯後的class文件如下:
//code6.class
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class Person {
private static final Logger log = LoggerFactory.getLogger(Person.class);
public Person() {
}
}
@Builder
@Builder作用在類上可以將類轉換爲建造者模式
//code7
import lombok.Builder;
@Builder
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
}
編譯後的class文件如下
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
Person(Long personId, String misNum, String fullName, String name) {
this.personId = personId;
this.misNum = misNum;
this.fullName = fullName;
this.name = name;
}
public static PersonBuilder builder() {
return new PersonBuilder();
}
}
實例化對象時可以按照下面的方式寫:
public class PersonTest {
public static void main(String[] args){
Person person = Person.builder().personId(1L).misNum("zhangsan").build();
}
}
也可以指定默認值:
import lombok.Builder;
import lombok.Singular;
import lombok.ToString;
@Builder
@ToString
public class Person {
private Long personId;
private String misNum;
private String fullName;
private String name;
@Builder.Default
private int tenant = 1;
}
其他的一些方法比較雞肋 就不一一列出了
原理
lombok的基本流程是:
定義編譯期的註解
利用JSR269 api(Pluggable Annotation Processing API )創建編譯期的註解處理器
利用tools.jar的javac api處理AST(抽象語法樹)
將功能註冊進jar包
因爲是在編譯期生效的 所以其實直接從代碼上來看,代碼可能都是錯誤的,所以需要安裝對應的IDE 插件對這些錯誤進行排除
想要了解lombok的原理,肯定是手擼代碼來的快
前邊提到lombok提供的toString方法不是json格式的,不如我們先來寫一個json格式的toString方法 姑且叫做ToJsonString
@ToJsonString
先定義註解類:
package lombok;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
//作用到類
@Target({ElementType.TYPE})
//只在編譯期起作用
@Retention(RetentionPolicy.SOURCE)
public @interface ToJsonString {
}
然後定義對應的處理器:
package lombok;
import com.sun.source.tree.Tree;
import com.sun.tools.javac.api.JavacTrees;
import com.sun.tools.javac.code.Flags;
import com.sun.tools.javac.code.TypeTag;
import com.sun.tools.javac.processing.JavacProcessingEnvironment;
import com.sun.tools.javac.tree.JCTree;
import com.sun.tools.javac.tree.TreeMaker;
import com.sun.tools.javac.tree.TreeTranslator;
import com.sun.tools.javac.util.*;
import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;
import java.util.Set;
@SupportedAnnotationTypes("lombok.ToJsonString")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class ToJsonStringProcessor extends AbstractProcessor {
//主要是用來在編譯期打log用的
private Messager messager;
//提供了待處理的抽象語法樹
private JavacTrees trees;
//封裝了創建AST節點的一些方法
private TreeMaker treeMaker;
//提供了創建標識符的方法
private Names names;
/**
* 從環境裏獲取一些關鍵信息
* @param processingEnv
*/
@Override
public synchronized void init(ProcessingEnvironment processingEnv) {
super.init(processingEnv);
this.messager = processingEnv.getMessager();
this.trees = JavacTrees.instance(processingEnv);
Context context = ((JavacProcessingEnvironment) processingEnv).getContext();
this.treeMaker = TreeMaker.instance(context);
this.names = Names.instance(context);
}
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
//獲取被ToJsonString標記的類
Set<? extends Element> set = roundEnv.getElementsAnnotatedWith(ToJsonString.class);
//遍歷 生成語法樹
set.forEach(element -> {
JCTree jcTree = trees.getTree(element);
jcTree.accept(new TreeTranslator() {
@Override
public void visitClassDef(JCTree.JCClassDecl jcClassDecl) {
List<JCTree.JCVariableDecl> jcVariableDeclList = List.nil();
for (JCTree tree : jcClassDecl.defs) {
if (tree.getKind().equals(Tree.Kind.VARIABLE)) {
JCTree.JCVariableDecl jcVariableDecl = (JCTree.JCVariableDecl) tree;
jcVariableDeclList = jcVariableDeclList.append(jcVariableDecl);
}
}
jcClassDecl.defs = jcClassDecl.defs.prepend(makeToJsonStringMethodDecl(jcVariableDeclList));
super.visitClassDef(jcClassDecl);
}
});
});
return false;
}
private JCTree.JCMethodDecl makeToJsonStringMethodDecl(List<JCTree.JCVariableDecl> jcVariableDeclList) {
//方法名
Name toString = names.fromString("toString");
//返回類型
JCTree.JCExpression returnType = getType("java.lang.String");
//泛型參數列表
List<JCTree.JCTypeParameter> methodGenericParams = List.nil();
//參數列表
List<JCTree.JCVariableDecl> params = List.nil();
//異常拋出列表
List<JCTree.JCExpression> throwsClauses = List.nil();
//拼接要輸出的JSON
JCTree.JCExpression jcExpression = treeMaker.Literal( "{");
for (int i = 0; i < jcVariableDeclList.size(); i++) {
JCTree.JCVariableDecl jcVariableDecl = jcVariableDeclList.get(i);
if (i != 0){
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Literal(",\"" + jcVariableDecl.name.toString() + "\":"));
}else{
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Literal("\""+jcVariableDecl.name.toString() + "\":"));
}
if (jcVariableDecl.vartype.toString().contains("String")){
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Literal("\""));
}
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Ident(jcVariableDecl.name));
if (jcVariableDecl.vartype.toString().contains("String")){
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Literal("\""));
}
}
jcExpression = treeMaker.Binary(JCTree.Tag.PLUS, jcExpression, treeMaker.Literal("}"));
JCTree.JCStatement jcStatement = treeMaker.Return(jcExpression);
List<JCTree.JCStatement> jcStatementList = List.nil();
jcStatementList = jcStatementList.append(jcStatement);
//方法體
JCTree.JCBlock body = treeMaker.Block(0, jcStatementList);
return treeMaker.MethodDef(treeMaker.Modifiers(Flags.PUBLIC),toString,returnType,List.nil(), params, List.nil(), body, null);
}
private JCTree.JCExpression getType(String components) {
String[] componentArray = components.split("\\.");
JCTree.JCExpression expr = treeMaker.Ident(names.fromString(componentArray[0]));
for (int i = 1; i < componentArray.length; i++) {
expr = treeMaker.Select(expr, names.fromString(componentArray[i]));
}
return expr;
}
}
再定義一個實體類,並使用@ToJsonString註解
@ToJsonString
public class Person {
private String misNum;
private String fullName;
private Long personId;
}
需要注意下 因爲現在不是jar依賴,並且本身是在編譯期生效,所以我們需要先編譯處理器 再編譯Person類,切換到代碼的文件目錄下,手動編譯
//首先創建一個文件夾 存放編譯後的class文件
mrdir lombokclass
//對編譯器進行編譯
javac -cp $JAVA_HOME/lib/tools.jar ToJsonString* -d lombokclass
//編譯Person類
javac -cp lombokclass -processor lombok.ToJsonStringProcessor Person.java -d lombokclass
我是直接在IDEA中看的編譯後的文件,如下圖:
關於JSR269 可以參考網上的這篇文章:JSR269參考資料
爭議
正所謂:愛的人愛的瘋狂 恨的人恨的發飆
lombok的好處顯而易見,代碼簡潔,看着舒服 擴展時不必重寫toString等
但反對使用的人也有很多正當理由
1.入侵性強 耦合增加 脅迫使用
首先如果你在項目裏使用了lombok,那同一個項目組的小夥伴都必須安裝lombok插件;如果你在對外提供的api裏使用了lombok,那引用你的依賴的項目也必須使用lombok,簡直是在耍流氓
2.技術債務
lombok自身無法及時支持最高的Java版本 如果升級項目的java版本時,lombok對此並不支持,將需要做很多工作去生成對應的方法
3.代碼調試困難
如果我們需要查看某個getter方法在哪些地方被引用 你會發現做不到
4.equals重寫
@Data默認對equals進行了重寫,很難說這是不是一件好事,如果只使用了@Data,而不使用@EqualsAndHashCode(callSuper=true)的話,會默認是@EqualsAndHashCode(callSuper=false),這時候生成的equals()方法只會比較子類的屬性,不會考慮從父類繼承的屬性。
所以,就我個人來說,不首先使用lombok,如果項目裏已經有了 那我就用