Problem
RESTEasy jackson provider讓我們可以輕鬆的將第三方服務回應的json內容,轉成我們自己的物件,讓程式碼只專注於資源物件的操作;然而,第三方服務並非永遠都如你預期,它可能突然更新而吐出無法處理的訊息:
javax.ws.rs.ProcessingException: RESTEASY003145: Unable to find a MessageBodyReader of content-type text/html;charset=iso-8859-1
我有幸遇到這樣的問題,也讓我思考著該如何將原始資料記錄下來以回報第三方服務提供者。本篇文章分享可能的作法,可依照你的需求自行挑選。
(完整程式內容可以從github上clone: link)
How to resolve?
ReadInterceptor & InputStreamSniffer
一開始遇到這個問題時,我的想法是: 是否可以透過interceptor去處理? 我所提供的第一個方法,是註冊一個ReadInterceptor,並且wrap原始的InputStream,將讀取的內容給記錄下來。首先是ReaderInterceptor,它會從Context中取出InputStream,並將其wrap成InputStreamSniffer,待讀取流程執行後,就可以透過getReadContent去拿到讀取的內容:
@Provider public class SnifferReadInterceptor implements ReaderInterceptor { private InputStreamSniffer inputStreamSniffer = null; @Override public Object aroundReadFrom(ReaderInterceptorContext context) throws IOException, WebApplicationException { InputStream inputStream = context.getInputStream(); inputStreamSniffer = new InputStreamSniffer(inputStream); context.setInputStream(inputStreamSniffer); return context.proceed(); } public String getReadContent(){ return inputStreamSniffer.getContent(); } }
InputStreamSniffer則是將讀取的動作delegate給原始的InputStream,並悄悄地記下讀取內容:
public class InputStreamSniffer extends InputStream { private InputStream srcInputStream; private StringBuilder contentStringBuilder; public InputStreamSniffer(InputStream inputStream){ srcInputStream = inputStream; contentStringBuilder = new StringBuilder(); } @Override public int read() throws IOException { int read_c = srcInputStream.read(); contentStringBuilder.append((char)read_c); return read_c; } // .. skip
我簡單的寫了一個測試,想要確認將message轉為json物件發生問題時,readInterceptor是否可以拿到message:
@Test public void testSnifferReadInterceptorWithReadFailed(){ Invocation invocation = initInvocation(); Response response = null; try { response = invocation.invoke(); response.readEntity(TestModel.class); fail("should be failed "); } catch( ProcessingException e ) { // Can't find valid messageBodyReader assertTrue(readInterceptor.getReadContent().isEmpty()); } finally { response.close(); } }
答案是不行,這是由於RESTEasy在讀取訊息前,會先根據Response Header中宣告的MediaType去找尋是否有對應的MessageBodyRead。
Read All Content of ReadInterceptor
由於我們無法於執行過程中解決這個問題,因此改在ReadInterceptor中,先將所有內容讀出來:
public class DebugReadInterceptor implements ReaderInterceptor{ private String content; @Override public Object aroundReadFrom(ReaderInterceptorContext context) throws IOException, WebApplicationException { InputStream inputStream = context.getInputStream(); content = IOUtils.toString(inputStream); context.setInputStream(new ByteArrayInputStream(content.getBytes())); return context.proceed(); } public String getContent(){ return content; } }
這個方法是可以解決目前遇到的問題,但缺點是每次請求都一定會多處理一次內容。於是我把腦筋動到了MessageBodyReader身上。
Customized MessageBodyReader
我新增一個名為CompositeMessageBodyProvider的類別,去implement MessageBodyReader;isReadable永遠回傳true,將搜尋合適MessageBodyReader動作交由readFrom處理,如果找不到就丟ProcessingException上去。Client可以攔截此例外決定要處理的方式:
@Provider @Consumes(MediaType.WILDCARD) @Produces(MediaType.WILDCARD) public class CompositeMessageBodyProvider implements MessageBodyReader<Object>, MessageBodyWriter<Object> { private List<MessageBodyReader<Object>> messageBodyReaders = new ArrayList<>(); @Override public boolean isReadable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType) { return true; } private Optional<MessageBodyReader<Object>> findReader(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType){ return messageBodyReaders.stream().filter(writter->writter.isReadable(type, genericType, annotations, mediaType)).findAny(); } @Override public Object readFrom(Class<Object> type, Type genericType, Annotation[] annotations, MediaType mediaType, MultivaluedMap<String, String> httpHeaders, InputStream entityStream) throws IOException, WebApplicationException { Optional<MessageBodyReader<Object>> reader = findReader(type, genericType, annotations, mediaType); if(reader.isPresent()){ return reader.get().readFrom(type, genericType, annotations, mediaType, httpHeaders, entityStream); } String output = IOUtils.toString(entityStream); throw new ProcessingException("Unexpected content: " + output); } // .. skip
Summary
以parse error、unexpected media type與unexpected json content(parse ok)三種錯誤來說:
- ReadInterceptor & InputStreamSniffer: 適用於parse error與unexpected json content。
- Read All Content of ReadInterceptor: 適用於以上提及的所有錯誤情形,但效率較差。
- Customized MessageBodyReader: 適用於unexpected media type。
要使用哪種方式,可仔細評估後再決定。
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