單個物件和物件串列的HttpMessageConverter

我有一個物件（此處：屬性），我想為我的 Spring 后端添加 csv 匯出功能，用于單個物件和物件串列。

我將此添加到我的配置中：

@Override
public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
    configurer.(more config).mediaType("csv", new MediaType("text", "csv"));
}

@Override
public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
     converters.add(new PropertyConverter());
     converters.add(new StatsConverter());
}

屬性轉換器如下所示：

public class PropertyConverter extends AbstractGenericHttpMessageConverter<Property> {

    private static final Logger LOGGER = LoggerFactory.getLogger(PropertyConverter.class);

    public PropertyConverter() {
        super(new MediaType("text", "csv"));
    }

    @Override
    protected void writeInternal(Property property, Type type, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        try (var writer = new OutputStreamWriter(outputMessage.getBody())) {
            new StatefulBeanToCsvBuilder<>(writer).withSeparator(',').build().write(property);
        } catch (CsvDataTypeMismatchException | CsvRequiredFieldEmptyException ex) {
            LOGGER.error("CSV failed to convert property: ".concat(property.getExternalId()).concat(", exception: ".concat(ex.toString())));
        }
    }

    @Override
    protected Property readInternal(Class<? extends Property> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        return null;
    }

    @Override
    public Property read(Type type, Class<?> contextClass, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        return null;
    }
}

此代碼適用于單個屬性。當我嘗試回傳屬性串列時：

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class eu.webeng.model.Property (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; eu.webeng.model.Property is in unnamed module of loader 'app')
    ...

Caused by: java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class eu.webeng.model.Property (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; eu.webeng.model.Property is in unnamed module of loader 'app')
    at eu.webeng.converter.PropertyConverter.writeInternal(PropertyConverter.java:20) ~[classes/:na]
    at org.springframework.http.converter.AbstractGenericHttpMessageConverter.write(AbstractGenericHttpMessageConverter.java:104) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:287) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:219) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
    ....

我試圖添加一個 PropertyListConverter，但它不適用于 Single Property。當我嘗試添加兩者時，正在使用添加的第一個轉換器。

如何使轉換器適用于單個和屬性串列（或任何物件）

uj5u.com熱心網友回復：

如果您嘗試將以下方法添加到您的（單一屬性）轉換器中，它是否有效？

    @Override
    public boolean canWrite(Type type, Class<?> clazz, MediaType mediaType) {
        return super.canWrite(clazz, mediaType) && clazz == Property.class;
    }

該類AbstractMessageConverterMethodProcessor遍歷所有已注冊的轉換器并跳過該canWrite方法回傳的任何轉換器false。我已經從類中改編了上面的方法，AbstractGenericHttpMessageConverter對要寫入的值的類添加了額外的檢查，因此它應該只用于寫出Property值。

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