使用RestTemplate请求请求aws s3下载文件时，如果传递的url字符串已经经过编码（例如斜杠为 %2F ），aws会返回报错 Error parsing the X-Amz-Credential parameter; the Credential is mal-formed。

原因是字符串直接传给 RestTemplate.exchange() （ getForObject() 等其他方法也都一样，都是在背后都是调用了 execute() 方法）的话就会导致二次编码。具体可以看源码：

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// RestTemplate.java
    
@Override
public <T> ResponseEntity<T> exchange(String url, HttpMethod method,
        @Nullable HttpEntity<?> requestEntity, Class<T> responseType, Object... uriVariables)
        throws RestClientException {

    RequestCallback requestCallback = httpEntityCallback(requestEntity, responseType);
    ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(responseType);
    return nonNull(execute(url, method, requestCallback, responseExtractor, uriVariables));
}

...

@Override
@Nullable
public <T> T execute(String uriTemplate, HttpMethod method, @Nullable RequestCallback requestCallback,
        @Nullable ResponseExtractor<T> responseExtractor, Object... uriVariables) throws RestClientException {

    URI url = getUriTemplateHandler().expand(uriTemplate, uriVariables);
    return doExecute(url, uriTemplate, method, requestCallback, responseExtractor);
}
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// DefaultUriBuilderFactory.java

@Override
public URI expand(String uriTemplate, Object... uriVars) {
    return uriString(uriTemplate).build(uriVars);
}

...

private class DefaultUriBuilder implements UriBuilder {

    ...

    @Override
    public URI build(Object... uriVars) {
        if (ObjectUtils.isEmpty(uriVars) && !CollectionUtils.isEmpty(defaultUriVariables)) {
            return build(Collections.emptyMap());
        }
        if (encodingMode.equals(EncodingMode.VALUES_ONLY)) {
            uriVars = UriUtils.encodeUriVariables(uriVars);
        }
        UriComponents uric = this.uriComponentsBuilder.build().expand(uriVars);
        return createUri(uric);
    }
    
    ...
    }
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// UriUtils.java

public static Object[] encodeUriVariables(Object... uriVariables) {
    return Arrays.stream(uriVariables)
            .map(value -> {
                String stringValue = (value != null ? value.toString() : "");
                return encode(stringValue, StandardCharsets.UTF_8);
            })
            .toArray();
}

可以看到最终来到了 build() 这里，而其中 encodeUriVariables 就是做了编码的操作。

解决办法也很简单，调用 exchange() 时不要传字符串，而是传 URI ：

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URI uri = URI.create(url);
ResponseEntity<String> response = restTemplate.exchange(uri, HttpMethod.GET, null, String.class);

源码中，首个参数为 URI 的 execute() 重载是不会发生编码操作，而是直接进行底层的 doExecute() ：

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// RestTemplate.java

@Override
@Nullable
public <T> T execute(URI url, HttpMethod method, @Nullable RequestCallback requestCallback,
        @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {

    return doExecute(url, null, method, requestCallback, responseExtractor);
}

另外，这个问题似乎在WebClient中也有：https://stackoverflow.com/questions/74389361/upload-file-to-aws-s3-using-presigned-url-via-spring-webclient-error-the-crede

参考：