java - 使用Gson以自定义方式将Date对象序列化为JSON(注释)

Question

我有以下类结构

class A {
  B b;
}

class B {
    @CustomDateFormat(
        format = "yyyy-MM-dd"
    )
    protected Date creationDate;
}

我想将 A 类的实例序列化为 JSON，输出 JSON 具有使用注释 @CustomDateFormat 中的值进行格式化的creationDate 字段。是否可以？最好使用 Gson。这将在 Android 上执行，因此 Java 8 不具备 ATM 功能。

提前感谢您的任何想法

Answer 1

Is it possible?

有点。不幸的是，Gson 几乎不支持自定义注解。但是，Gson 对其 @JsonAdapter 注释提供 native 支持，以便您可以模拟自定义注释。

比方说，

final class A {

    final B b;

    A(final B b) {
        this.b = b;
    }

}

final class B {

    // Here comes an emulation for @CustomDateFormat(format = "yyyy-MM-dd")
    @JsonAdapter(YyyyMmDdDateTypeAdapter.class)
    final Date creationDate;

    B(final Date creationDate) {
        this.creationDate = creationDate;
    }

}

abstract class AbstractDateTypeAdapter
        extends TypeAdapter<Date> {

    protected abstract DateFormat getDateFormat();

    @Override
    @SuppressWarnings("resource")
    public final void write(final JsonWriter out, final Date value)
            throws IOException {
        out.value(getDateFormat().format(value));
    }

    @Override
    public final Date read(final JsonReader in) {
        throw new UnsupportedOperationException("Not implemented");
    }

}

final class YyyyMmDdDateTypeAdapter
        extends AbstractDateTypeAdapter {

    // Let Gson do it itself when needed
    private YyyyMmDdDateTypeAdapter() {
    }

    @Override
    protected DateFormat getDateFormat() {
        // SimpleDateFormat is known to be thread-unsafe so it has to be created everytime it's necessary
        // Maybe Joda Time is an option for you?
        // Joda Time date formatters are thread-safe and can be safely instantiated once per application
        return new SimpleDateFormat("yyyy-MM-dd");
    }

}

示例:

private static final Gson gson = new Gson();

public static void main(final String... args) {
    final A a = new A(new B(new Date()));
    final String json = gson.toJson(a);
    System.out.println(json);
}

输出:

{"b":{"creationDate":"2017-05-29"}}