How to run code after constructor in a Lombok builder

问题

I have a class that I want to use Lombok.Builder and I need pre-process of some parameters. Something like this:

@Builder
public class Foo {
   public String val1;
   public int val2;
   public List<String> listValues;

   public void init(){
       // do some checks with the values.
   }
}

normally I would just call init() on a NoArg constructor, but with the generated builder I'm unable to do so. Is there a way for this init be called by the generated builder? For example build() would generate a code like:

public Foo build() {
   Foo foo = Foo(params....)
   foo.init();
   return foo;
}

I'm aware that I can manually code the all args constructor, that the Builder will call through it and I can call init inside there.

But that is a sub-optimal solution as my class will likely have new fields added every once in a while which would mean changing the constructor too.

回答1:

After much trial and end error I found a suitable solution: extend the generate builder and call init() myself.

Example:

@Builder(toBuilder = true, builderClassName = "FooInternalBuilder", builderMethodName = "internalBuilder")
public class Foo {

   public String val1;
   public int val2;
   @Singular public List<String> listValues;

   void init() {
      // perform values initialisation
   }

   public static Builder builder() {
      return new Builder();
   }

   public static class Builder extends FooInternalBuilder {

      Builder() {
         super();
      }

      @Override public Foo build() {
         Foo foo = super.build();
         foo.init();
         return foo;
      }
   }
}

回答2:

In Foo you could manually add a constructor, have that do the initialization, and put @Builder on the constructor. I know that you already know this, but I think it is the right solution, and you won't forget to add the parameter since you do want to use the code in the builder anyway.

Disclosure: I am a lombok developer.

回答3:

I just stumbled upon the same issue. But additionally, I wanted to add an method buildOptional() to the builder to not repeat Optional.of(Foo) each time I need it. This did not work with the approach posted before because the chained methods return FooInternalBuilder objects; and putting buildOptional() into FooInternalBuilder would miss the init() method execution in Builder...

Also, I personally did not like the presence of 2 builder classes.

Here is what I did instead:

@Builder(buildMethodName = "buildInternal")
@ToString
public class Foo {
    public String val1;
    public int val2;
    @Singular  public List<String> listValues;

    public void init(){
        // do some checks with the values.
    }    

    /** Add some functionality to the generated builder class */
    public static class FooBuilder {
        public Optional<Foo> buildOptional() {
            return Optional.of(this.build());
        }

        public Foo build() {
            Foo foo = this.buildInternal();
            foo.init();
            return foo;
        }
    }
}

You can do a quick test with this main method:

public static void main(String[] args) {
    Foo foo = Foo.builder().val1("String").val2(14)
            .listValue("1").listValue("2").build();
    System.out.println(foo);

    Optional<Foo> fooOpt = Foo.builder().val1("String").val2(14)
            .listValue("1").listValue("2").buildOptional();
    System.out.println(fooOpt);
}

Doing so let's you add what I want:

Add an init() method which is executed after each object construction automatically
Adding new fields do not require additional work (as it would be for an individually written constructor)
Possibility to add additional functionality (incl. the init() execution)
Retain the complete standard functionality the @Builder annotation brings
Don't expose an additional builder class

Even if you solved your problem before I like to share this as the solution. It is a bit shorter and adds a (for me) nice feature.

回答4:

This works for me, not a complete solution, but quick and easy.

@Builder
@AllArgsConstructor
public class Foo {
   @Builder.Default
   int bar = 42;
   Foo init() {
      // perform values initialisation
     bar = 451;   // replaces 314
     return foo;
   }
   static Foo test() {
       return new FooBuilder()  // defaults to 42
           .bar(314)  // replaces 42 with 314
           .build()
           .init();   // replaces 314 with 451
   }
}

来源：https://stackoverflow.com/questions/37968696/how-to-run-code-after-constructor-in-a-lombok-builder

标签

java

Lombok