Lombok makes ⽤ @data and @builder's pit at the same time. Are you hit?

The problem background

Lombok Use Use at the same time @Data and @Builder , An error is reported when constructing a parameterless constructor ！ Compile not pass . Here's the picture ：

Lombok @Data and @Builder Analyze usage separately

Lombok Use @Data Sure ⽣ become ⽆ Participate in constructing classes ⾥⾯ All attributes of getter/setter Method . It can simplify the development of our code .（ Need to install Lombok Plug ins and leads ⼊Lombok rely on ）.

For example, next ⾯ Of ⼀ Entity classes , lead ⼊Lombok after , Sure ⾃ dynamic ⽣ become GET/SET Methods and ⽆ Parameter constructor .

The compiled class by : You can see that it not only helps us generate get and set , There is also a default parameterless constructor

So how to automatically generate a parameter constructor ？ Use @Builder annotation , Will help us ⽣ The construction method of completion attribute .

The compiled class by ： You can see It has helped us build a full attribute constructor , But if the value only references @Builder Annotations cannot be generated get and set Of . in addition , public Number Java selected , reply java interview , Access to interview information .

But if used at the same time @Data and @Builder Words , It can be seen that although ⽣ a GET/SET Method , however ⽆ The parameter construction method is gone , This is obviously unacceptable , Because many frameworks call ⽆ Reference structure to create objects .

The compiled class：

We try to be in Tet1 class ,⼿ Dynamic add ⽆ Refer to the construction method . The compiler finds that an error is reported and fails ：

resolvent

Method 1

Lombok Use at the same time @Data and @Builder When , If you want to ⽣ become ⽆ Reference structure , Need to be in code ⾥⾯⼿ Dynamic leading ⼊ annotation @Tolerate, Give Way Lombok stay ⽣ When you are in a class , Not aware of the specified constructor .

Method 2

Use the parameterless constructor directly + There is a way of involving constructors ,@RequiredArgsConstructor  To build a reference ,@NoArgsConstructor To build a parameterless constructor , As shown in the figure ：

The effect after compiling ：

Lombok principle

Java The compilation of is divided into the following ⼏ Stages ：

Parsing and filling symbol tables -> Annotation Processing -> Analysis and bytecode ⽣ become ->⽣ become ⼆ Base number class⽂ Pieces of .

• Lombok It uses JDK 6 Realized  JSR 269: Pluggable Annotation Processing API ( Compile time annotation processor ), It is in compile time to put Lombok The annotation code for , Convert to regular Java Method ⽽ Implementation note ⼊.

• At compile time , When Java Source code is abstract idiom tree (AST) after ,Lombok Will be based on ⾃⼰ The annotation processor dynamically modifies AST, Add new code ( node ), Here ⼀ Cut hold ⾏ after , And then through the analysis ⽣ It becomes the final bytecode (.class) ⽂ Pieces of , This is it. Lombok The hold of ⾏ principle .

It can be implemented with the help of annotation processor ⼀ A simple one. Setter, Our implementation steps are :

• Customize ⼀ An annotation tag is attached to ⼝, And implement ⼀ individual ⾃ Defined annotation handler ;

• utilize tools.jar Of javac api Handle AST ( Abstract syntax tree )3. Use ⾃ The defined annotation handler compiles the code .

1. Define custom annotations and annotation handlers

⾸ First create ⼀ individual  MySetter.java ⾃ Definition ⼀ A note , The code is as follows ：

Re realization ⼀ individual ⾃ Defined annotation handler , The code is as follows ：

The test class is as follows :

2. To the annotation processor ⾏ compile , Then use the annotation processor to enter the class ⾏ compile

⾸ The annotation processor needs to be modified first ⾏ compile （javac -cp For leading ⼊ Third jar Baojin ⾏ compile ）

Then use the annotation processor to process this Person Test class into ⾏ compile :

Let's look at it later ⽣ Yes Person.class, You can find Setter The method has been ⽣ a :

summary

Of course, although the test class has ⽣ become Setter Method , But because it is at compile time ⽣ Yes , Therefore, we cannot call directly when developing Setter Methodical , therefore Lombok Provides plug-in mechanism , We can call directly when developing Lombok Characteristics of .

author ： Shudongjun

https://juejin.cn/post/7103011031672176677

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