HttpClient: Content-Length header already present解決及分析

在用HttpClient的時候出現如下問題：

org.apache.http.client.ClientProtocolException
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:822)
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:754)
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:732)
……
Caused by: org.apache.http.ProtocolException: Content-Length header already present
at org.apache.http.protocol.RequestContent.process(RequestContent.java:67)
at org.apache.http.protocol.ImmutableHttpProcessor.process(ImmutableHttpProcessor.java:108)
at org.apache.http.protocol.HttpRequestExecutor.preProcess(HttpRequestExecutor.java:174)
at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:462)
at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:820)
... 27 more

• 出現上面的問題是發送請求的時候設置了請求頭Content-Length參數。
• 解決辦法：把設置的請求頭Content-Length參數去掉。
• 分析原因：
  跟蹤代碼:查看HttpClient中RequestContent類的process方法，看到當body非空時，會自動加上Content-Length請求頭及其對應值，不需要自己手動加上它。加上就會拋出上面的異常。
  源碼：

public class RequestContent implements HttpRequestInterceptor {
    private final boolean overwrite;

    public RequestContent() {
        this(false);
    }

    public RequestContent(boolean overwrite) {
        this.overwrite = overwrite;
    }

    public void process(HttpRequest request, HttpContext context) throws HttpException, IOException {
        Args.notNull(request, "HTTP request");
        if (request instanceof HttpEntityEnclosingRequest) {
            if (this.overwrite) {
                request.removeHeaders("Transfer-Encoding");
                request.removeHeaders("Content-Length");
            } else {
                if (request.containsHeader("Transfer-Encoding")) {
                    throw new ProtocolException("Transfer-encoding header already present");
                }

                if (request.containsHeader("Content-Length")) {
                    throw new ProtocolException("Content-Length header already present");
                }
            }

            ProtocolVersion ver = request.getRequestLine().getProtocolVersion();
            HttpEntity entity = ((HttpEntityEnclosingRequest)request).getEntity();
            if (entity == null) {
                request.addHeader("Content-Length", "0");
                return;
            }

            if (!entity.isChunked() && entity.getContentLength() >= 0L) {
            	//計算請求頭的長度
                request.addHeader("Content-Length", Long.toString(entity.getContentLength()));
            } else {
                if (ver.lessEquals(HttpVersion.HTTP_1_0)) {
                    throw new ProtocolException("Chunked transfer encoding not allowed for " + ver);
                }

                request.addHeader("Transfer-Encoding", "chunked");
            }

            if (entity.getContentType() != null && !request.containsHeader("Content-Type")) {
                request.addHeader(entity.getContentType());
            }

            if (entity.getContentEncoding() != null && !request.containsHeader("Content-Encoding")) {
                request.addHeader(entity.getContentEncoding());
            }
        }

    }
}