使用Step Builder创建Tebuddy

Question

我试图让ByteBuddy实现一个步骤构建器，给定该构建器的接口。我被困在两个地方。

1. 如何创建一个setter来返回方法链接的当前实例

我从以下几点开始：

.method(ElementMatchers.isSetter())
.intercept(FieldAccessor.ofBeanProperty());

只有我想返回当前的构建器实例，这样我们就可以像这样链接调用：

final Object obj = ...builder().id(100).name("test").build();

因此，我创建了一个这样的拦截器，看起来像是一个黑客，我希望尽可能避免反射：

@RuntimeType
public Object intercept(@RuntimeType Object arg, @This Object source, @Origin Method method)
{
  try
  {
    // Field name is same as method name.
    final Field field = source.getClass().getDeclaredField(method.getName());
    field.setAccessible(true);
    field.set(source, arg);
  }
  catch (Throwable ex)
  {
    throw new Error(ex);
  }

  // Return current builder instance.
  return source;
}

目前，我在一个循环中将字段添加到生成器类中，我在生成器上的构建方法被截取如下：

private static final class InterBuilder
{
  private final Collection<String> fields;
  private final Constructor<?> constructor;

  InterBuilder(final Constructor<?> constructor, final Collection<String> fields)
  {
    this.constructor = constructor;
    this.fields = fields;
  }

  @RuntimeType
  public Object intercept(@This Object source, @Origin Method method)
  {
    try
    {
      final Object[] args = Arrays.stream(source.getClass().getDeclaredFields())
        .filter(f -> this.fields.contains(f.getName()))
        .map(f ->  { try {
          f.setAccessible(true);
          return f.get(source); }
          catch (Throwable ex) { throw new Error(ex); } })
        .toArray();

      // Invoke a constructor passing in the private field values from the builder...
      return this.constructor.newInstance(args);
    }
    catch (Throwable ex)
    {
      throw new Error(ex);
    }
  }
}

我看到了@FieldValue注释。我不认为有什么东西会在不知道它们的名字的情况下给我所有字段？

该代码在这一点上是一个概念证明。有更好的方法来做我在这里做的事情吗<谢谢！

Answer 1

拉斐尔给了我想出解决方案所需的信息，所以答案归功于他，但我想把我的解决方案包括在内，让其他人在未来发现。

DynamicType.Builder<?> builder = new ByteBuddy()
  .subclass(Object.class)
  .implement(interfaces)
  .name(builderClassName);

// Find all of the setters on the builder...  
// Here I'm assuming all field names match setter names like:
//   MyClass x = theBuilder.name("hi").id(1000).isValid(true).build();
final List<Method> setters = ...

for (final Method setter : setters)
{
  // This will define a field and a setter that will set the value and return the current instance.
  builder = builder
    .defineField(setter.getName(), setter.getParameterTypes()[0], Visibility.PRIVATE)
    .define(setter)
    .intercept(FieldAccessor.ofField(setter.getName()).setsArgumentAt(0).andThen(FixedValue.self()));
}

// Find the "build" method on the builder.
final Method buildMethod = ...

// Get a constructor that you want the builder to call and return the new instance.
final Constructor<?> constructor = ...

// Get the field names from the setters.
final List<String> fieldNames = setters.stream()
  .map(Method::getName)
  .collect(Collectors.toList());

// This will define a "build" method that will invoke the constructor of some object and
// pass in the fields (in order) of the builder to that constructor.
builder = builder
  .define(buildMethod)
  .intercept(MethodCall.construct(constructor)
    .withField(fieldNames.toArray(new String[fieldNames.size()])));

Answer 2

您可以组成两个实现：

FieldAccessor.ofBeanProperty().setsArgumentAt(0).andThen(FixedValue.self());

这将首先设置setters（索引0）参数，然后返回This。

如果您想在没有反射的情况下从Method odEnxy设置一个字段，请查看FieldProxy。