Get和Post各種編碼方式和獲取參數的問題
做後端開發，會跟各種請求打交道，Get或者Post甚至其他，而就Get和Post不同的編碼方式和content-type提交也有許多的不同，在開發過程中，經常遇到參數接收不到的問題，無論是原生servlet還是springboot框架，都有到過這種問題。這篇文中就這些問題，通過分析，希望能讓自己能更進一步的瞭解HTTP請求和參數接收相關的原理。
Get請求
Get請求比較簡單，請求的參數爲[key=value]，通過&連接起來，一般被稱爲查詢字符串(Query String)，例如:http://127.0.0.1/test?name=strive&age=23，參數爲:name=strive&age=23.
GET /get?name=strive HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Sec-Fetch-Site: same-origin
Referer: http://localhost:8080/getAndPostIndex
Accept-Encoding: gzip, deflate, br
其實我們只需要看/get?name=strive即查詢參數，GET請求參數是被存放在QueryString 中的，這種在我們後臺是可以通過request.getParameter() 獲取請求參數的。
POST請求
下面我們來看POST請求，POST請求的方式比Get複雜，它參數的存放位置是和Content-Type 有關係的，具體有什麼關係，繼續往下看。
Content-Type爲application/x-www-form-urlencoded
POST /postUrl HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Content-Length: 11
Cache-Control: max-age=0
Origin: http://localhost:8080
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Sec-Fetch-Site: same-origin
Referer: http://localhost:8080/getAndPostIndex
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,ko;q=0.8,en;q=0.7
Form Data
name=strive
這種方式提交的參數放在了以[key=value]方式存在了請求體中，這種在我們後臺是也是可以通過request.getParameter() 獲取請求參數的。
Content-Type爲multipart/form-data
POST /postFileUpload HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Content-Length: 3768
Cache-Control: max-age=0
Origin: http://localhost:8080
Upgrade-Insecure-Requests: 1
#Content-Type爲multipart/form-data
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary6A2xLxBKaSvqBvDd
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Sec-Fetch-Site: same-origin
Referer: http://localhost:8080/getAndPostIndex
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,ko;q=0.8,en;q=0.7
#Request的請求體
Request Payload
------WebKitFormBoundary6A2xLxBKaSvqBvDd
Content-Disposition: form-data; name="file"; filename="Baidu.cer"
Content-Type: application/x-x509-ca-cert
#這裏省略了文件的數據
------WebKitFormBoundary6A2xLxBKaSvqBvDd
Content-Disposition: form-data; name="name"
strive
------WebKitFormBoundary6A2xLxBKaSvqBvDd--
在Content-Type: multipart/form-data 時，而且POST提交的參數存放在了Request Payload，這種請求參數沒辦法通過request.getParameter() 獲取請求參數的。
Content-Type爲application/json
POST /postJson HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Content-Length: 13
Accept: */*
Origin: http://localhost:8080
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
Sec-Fetch-Mode: cors
Content-Type: application/json
Sec-Fetch-Site: same-origin
Referer: http://localhost:8080/getAndPostIndex
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,ko;q=0.8,en;q=0.7
#Request的請求體
Request Payload
{name:strive}
在Content-Type: application/json 時，而且POST提交的參數存放在了Request Payload，這種請求參數沒辦法通過request.getParameter() 獲取請求參數的。
Content-Type爲text/plain
POST /postPlain HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Content-Length: 13
Cache-Control: max-age=0
Origin: http://localhost:8080
Upgrade-Insecure-Requests: 1
Content-Type: text/plain
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Sec-Fetch-Site: same-origin
Referer: http://localhost:8080/getAndPostIndex
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9,ko;q=0.8,en;q=0.7
#Request的請求體
Request Payload
name=strive
在Content-Type: text/plain 時，而且POST提交的參數存放在了Request Payload，這種請求參數沒辦法通過request.getParameter() 獲取請求參數的。
POST請求的問題
從上面的幾個請求的報文的例子中我們可以找到一些規律，以application/x-www-form-urlencoded作POST提交的數據，後臺是可以通過request.getParameter() 獲取請求參數的。而multipart/form-data,application/json和text/plain作POST提交數據時，後臺沒法通過request.getParameter() 獲取請求參數的。application/x-www-form-urlencoded的參數放在了名爲Form Data的請求體中，而其他的則放在了名爲Request Payload的請求體中。那是什麼原因了？
解析請求參數的實現源碼org.apache.catalina.connector.Request#parseParameters如下：
protected void parseParameters() {
this.parametersParsed = true;
Parameters parameters = this.coyoteRequest.getParameters();
boolean success = false;
try {
parameters.setLimit(this.getConnector().getMaxParameterCount());
Charset charset = this.getCharset();
boolean useBodyEncodingForURI = this.connector.getUseBodyEncodingForURI();
parameters.setCharset(charset);
if (useBodyEncodingForURI) {
parameters.setQueryStringCharset(charset);
}
parameters.handleQueryParameters();
if (this.usingInputStream || this.usingReader) {
success = true;
return;
}
//獲取請求的contentType
String contentType = this.getContentType();
if (contentType == null) {
contentType = "";
}
int semicolon = contentType.indexOf(59);
if (semicolon >= 0) {
contentType = contentType.substring(0, semicolon).trim();
} else {
contentType = contentType.trim();
}
//處理文件上傳
if ("multipart/form-data".equals(contentType)) {
this.parseParts(false);
success = true;
return;
}
if (!this.getConnector().isParseBodyMethod(this.getMethod())) {
success = true;
return;
}
if ("application/x-www-form-urlencoded".equals(contentType)) {
//如果是這種contenttype則解析
int len = this.getContentLength();
if (len <= 0) {
if ("chunked".equalsIgnoreCase(this.coyoteRequest.getHeader("transfer-encoding"))) {
Object var21 = null;
Context context;
byte[] formData;
try {
formData = this.readChunkedPostBody();
} catch (IllegalStateException var17) {
parameters.setParseFailedReason(FailReason.POST_TOO_LARGE);
context = this.getContext();
if (context != null && context.getLogger().isDebugEnabled()) {
context.getLogger().debug(sm.getString("coyoteRequest.parseParameters"), var17);
}
return;
} catch (IOException var18) {
parameters.setParseFailedReason(FailReason.CLIENT_DISCONNECT);
context = this.getContext();
if (context != null && context.getLogger().isDebugEnabled()) {
context.getLogger().debug(sm.getString("coyoteRequest.parseParameters"), var18);
}
return;
}
if (formData != null) {
parameters.processParameters(formData, 0, formData.length);
}
}
} else {
int maxPostSize = this.connector.getMaxPostSize();
Context context;
if (maxPostSize >= 0 && len > maxPostSize) {
context = this.getContext();
if (context != null && context.getLogger().isDebugEnabled()) {
context.getLogger().debug(sm.getString("coyoteRequest.postTooLarge"));
}
this.checkSwallowInput();
parameters.setParseFailedReason(FailReason.POST_TOO_LARGE);
return;
}
context = null;
byte[] formData;
if (len < 8192) {
if (this.postData == null) {
this.postData = new byte[8192];
}
formData = this.postData;
} else {
formData = new byte[len];
}
try {
if (this.readPostBody(formData, len) != len) {
parameters.setParseFailedReason(FailReason.REQUEST_BODY_INCOMPLETE);
return;
}
} catch (IOException var19) {
Context context = this.getContext();
if (context != null && context.getLogger().isDebugEnabled()) {
context.getLogger().debug(sm.getString("coyoteRequest.parseParameters"), var19);
}
parameters.setParseFailedReason(FailReason.CLIENT_DISCONNECT);
return;
}
// 處理POST請求參數，把它放到requestparameter map中（即request.getParameterMap獲取到的Map，request.getParameter(name)也是從這個Map中獲取的）
parameters.processParameters(formData, 0, len);
}
success = true;
return;
}
success = true;
} finally {
if (!success) {
parameters.setParseFailedReason(FailReason.UNKNOWN);
}
}
}