使用HttpURLConnection 上传大文件，会出现内存溢出问题：

观察HttpURLConnection 源码：

@Overridepublic synchronized OutputStream getOutputStream() throws IOException {connecting = true;SocketPermission p = URLtoSocketPermission(this.url);if (p != null) {try {return AccessController.doPrivilegedWithCombiner(new PrivilegedExceptionAction<>() {public OutputStream run() throws IOException {return getOutputStream0();}}, null, p            );} catch (PrivilegedActionException e) {throw (IOException) e.getException();}} else {return getOutputStream0();}
}

private synchronized OutputStream getOutputStream0() throws IOException {try {if (!doOutput) {throw new ProtocolException("cannot write to a URLConnection"                           + " if doOutput=false - call setDoOutput(true)");}if (method.equals("GET")) {method = "POST"; // Backward compatibility        }if ("TRACE".equals(method) && "http".equals(url.getProtocol())) {throw new ProtocolException("HTTP method TRACE" +" doesn't support output");}// if there's already an input stream open, throw an exception        if (inputStream != null) {throw new ProtocolException("Cannot write output after reading input.");}if (!checkReuseConnection())connect();boolean expectContinue = false;String expects = requests.findValue("Expect");if ("100-Continue".equalsIgnoreCase(expects) && streaming()) {http.setIgnoreContinue(false);expectContinue = true;}if (streaming() && strOutputStream == null) {writeRequests();}if (expectContinue) {expect100Continue();}ps = (PrintStream)http.getOutputStream();if (streaming()) {if (strOutputStream == null) {if (chunkLength != -1) { /* chunked */                     strOutputStream = new StreamingOutputStream(new ChunkedOutputStream(ps, chunkLength), -1L);} else { /* must be fixed content length */                    long length = 0L;if (fixedContentLengthLong != -1) {length = fixedContentLengthLong;} else if (fixedContentLength != -1) {length = fixedContentLength;}strOutputStream = new StreamingOutputStream(ps, length);}}return strOutputStream;} else {if (poster == null) {poster = new PosterOutputStream();}return poster;}} catch (RuntimeException e) {disconnectInternal();throw e;} catch (ProtocolException e) {// Save the response code which may have been set while enforcing        // the 100-continue. disconnectInternal() forces it to -1        int i = responseCode;disconnectInternal();responseCode = i;throw e;} catch (IOException e) {disconnectInternal();throw e;}
}

public boolean streaming () {return (fixedContentLength != -1) || (fixedContentLengthLong != -1) ||(chunkLength != -1);
}

如上， 默认设置情况下streaming ()  为false。

package sun.net.www.http
public class PosterOutputStream extends ByteArrayOutputStream {
}

PosterOutputStream  默认为 ByteArrayOutputStream  子类

解决办法：

目标服务支持情况下，可以不使用HttpURLConnection的ByteArrayOutputStream缓存机制，直接将流提交到服务器上。如下函数设置：

httpConnection.setChunkedStreamingMode(0); // 或者设置自定义大小，0默认大小

    public void setChunkedStreamingMode (int chunklen) {if (connected) {throw new IllegalStateException ("Can't set streaming mode: already connected");}if (fixedContentLength != -1 || fixedContentLengthLong != -1) {throw new IllegalStateException ("Fixed length streaming mode set");}chunkLength = chunklen <=0? DEFAULT_CHUNK_SIZE : chunklen;}

遗憾的是，我上传的服务不支持这种模式。因此采用固定大小。

HttpURLConnection con = (HttpURLConnection)new URL("url").openConnection();
con.setFixedLengthStreamingMode(输出流的固定长度);

  /*** This method is used to enable streaming of a HTTP request body* without internal buffering, when the content length is known in* advance.* <p>* An exception will be thrown if the application* attempts to write more data than the indicated* content-length, or if the application closes the OutputStream* before writing the indicated amount.* <p>* When output streaming is enabled, authentication* and redirection cannot be handled automatically.* A HttpRetryException will be thrown when reading* the response if authentication or redirection are required.* This exception can be queried for the details of the error.* <p>* This method must be called before the URLConnection is connected.* <p>* <B>NOTE:</B> {@link #setFixedLengthStreamingMode(long)} is recommended* instead of this method as it allows larger content lengths to be set.** @param   contentLength The number of bytes which will be written*          to the OutputStream.** @throws  IllegalStateException if URLConnection is already connected*          or if a different streaming mode is already enabled.** @throws  IllegalArgumentException if a content length less than*          zero is specified.** @see     #setChunkedStreamingMode(int)* @since 1.5*/public void setFixedLengthStreamingMode (int contentLength) {if (connected) {throw new IllegalStateException ("Already connected");}if (chunkLength != -1) {throw new IllegalStateException ("Chunked encoding streaming mode set");}if (contentLength < 0) {throw new IllegalArgumentException ("invalid content length");}fixedContentLength = contentLength;}

我是上传文件场景： 使用文件的大小作为长度

FileInputStream fileInputStream = new FileInputStream(uploadFileName);
long totalLength= fileInputStream.getChannel().size();
var boundary = "someboundary";
var temUploadUrl = "url path";
//
var url = new URL(temUploadUrl);
var connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("PUT");
connection.setRequestProperty("Content-Type", MediaType.MULTIPART_FORM_DATA_VALUE + "; boundary=" + boundary);connection.setDoOutput(true);
// 设置 Content-Length
connection.setRequestProperty("Content-Length", String.valueOf(totalLength)); 
connection.setFixedLengthStreamingMode(totalLength);