@在JAXRS/RestEasy中产生集合 [英] @Produces collection in JAXRS / RestEasy

问题描述

我发现了一些我无法理解的奇怪行为.

I found some strange behaviour that I cannot understand.

我已经测试了4个类似的示例:

I have tested 4 similar examples:

@GET
@Produces(MediaType.APPLICATION_JSON)
public Response produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return Response.ok(books).build();
}

2

@GET
@Produces(MediaType.APPLICATION_JSON)
public List<Book> produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return books;
}

3

@GET
@Produces(MediaType.APPLICATION_XML)
public List<Book> produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return books;
}

4

@GET
@Produces(MediaType.APPLICATION_XML)
public Response produce() {
    List<Book> books = Arrays
            .asList(new Book[] { 
                    new Book("aaa", "AAA", "12345"), 
                    new Book("bbb", "BBB", "09876") 
                    });
    return Response.ok(books).build();
}

一切都在#1，#2，#3中起作用，但第4个示例抛出:

Everything works in #1, #2, #3 but 4th example throws:

找不到类型为的响应对象的MessageBodyWriter: 媒体类型为java.util.Arrays $ ArrayList:application/xml.

Could not find MessageBodyWriter for response object of type: java.util.Arrays$ArrayList of media type: application/xml.

我在Wildfly 9上运行它，不知道它是否与RestEasy或JaxRS有关?我知道可以通过将集合包装在GenericEntity中来解决此问题，但是我不理解这种不一致的行为.

I run it on Wildfly 9 and I wonder if it is related to RestEasy or JaxRS in general? I know that I can fix it by wrapping collection in GenericEntity, but I don't understand this inconsistent behaviour.

推荐答案

问题是缺少类型信息.对于处理XML序列化的JAXB，这是必需的.

The problem is the missing of type information. This is required for JAXB, which handles the XML serialization.

1和2之所以起作用，是因为Jackson用于JSON，并且通常不需要内省属性就知道类型信息.

1 and 2 works because Jackson is being used for JSON, and it generally doesn't need to know type information as it just introspects properties.

3之所以有效，是因为通过方法返回类型可以知道类型信息.

3 works because type information is known through the method return type.

4不起作用，因为没有类型信息. 类型擦除将其删除.这就是GenericEntity的解决方法.它存储类型信息.

4 doesn't work because there is no type information. It's is erased by type erasure. That's where GenericEntity comes to the rescue. It stores the type information.

GenericEntity

通常，类型擦除会删除通用类型信息，以使包含例如类型为List<String>的实体的Response实例在运行时看起来包含原始的List<?>.当需要通用类型来选择合适的MessageBodyWriter时，可以使用此类包装实体并捕获其通用类型.

Normally type erasure removes generic type information such that a Response instance that contains, e.g., an entity of type List<String> appears to contain a raw List<?> at runtime. When the generic type is required to select a suitable MessageBodyWriter, this class may be used to wrap the entity and capture its generic type.

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