当contentType为application/json的时候，在servlet中通过request.getParameter得到的数据为空。今天我们就java的请求，分析一下request得到参数的过程。如果你还不知道自己喜欢什么，你就真的迷失了。
java的请求参数
首先我把环境代码贴出来，如下前端jsp的代码：$.ajax方法里面默认的contenttype为：application/x-www-form-urlencoded
<script type="text/javascript" src="js/jquery-3.1.0.js"></script>
<script type="text/javascript">
function testJson() {
$.ajax({
url: "ParameterServlet",
type: "post",
data: {
"username": "huhx",
"love": "javascript"
}
// contentType: "application/json"
});
}
</script>
</head>
<body>
hello world.
<input type="text" name="username"><br>
<button onclick="testJson()">love you, huhx</button>
</body>
</html>
后端java的代码：
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
System.out.println(request.getContentType());
String username = request.getParameter("username");
System.out.println(username);
}
一、request.getParameter("username")执行的实际代码其实是Request类的getParameter方法：
public String getParameter(String name) {
if (!parametersParsed) {
parseParameters();
}
return coyoteRequest.getParameters().getParameter(name);
}
parametersParsed是一个为false的全局变量，由于是线程的问题。parseParameters()针对这个请求应该只会执行一次。这里关键代码有两个，其一如下：
String contentType = getContentType();
if (contentType == null) {
contentType = "";
}
int semicolon = contentType.indexOf(';');
if (semicolon >= 0) {
contentType = contentType.substring(0, semicolon).trim();
} else {
contentType = contentType.trim();
} if ("multipart/form-data".equals(contentType)) {
parseParts();
success = true;
return;
} if (!("application/x-www-form-urlencoded".equals(contentType))) {
success = true;
return;
}
这段代码不难理解，首先会得到请求类型。从上述代码可以得出两点：
- 当contenttype有多种时，只会取第一种。比如contenttype为application/json;application/x-www-form-urlencoded，最终是application/json
- 当contenttype不为application/x-www-form-urlencoded，直接return掉了。也就是说关键代码二不执行了。
- 当contenttype为multipart/form-data时，parseParts()方法里使用的解析文件的框架是apache自带的fileupload。
好了，以下贴出关键二的代码：
formData = readChunkedPostBody();
parameters.processParameters(formData, 0, len)
- readChunkedPostBody方法，其实就是request.getInputStream().read()方法，将请求的数据通过流的方式读取出来。为了方便，下面贴出代码：
protected byte[] readChunkedPostBody() throws IOException {
ByteChunk body = new ByteChunk();
byte[] buffer = new byte[CACHED_POST_LEN];
int len = 0;
while (len > -1) {
len = getStream().read(buffer, 0, CACHED_POST_LEN); // getStream()返回的是CoyoteInputStream，一种带有缓存的流。
if (connector.getMaxPostSize() >= 0 &&
(body.getLength() + len) > connector.getMaxPostSize()) {
// Too much data
checkSwallowInput();
throw new IllegalStateException(
sm.getString("coyoteRequest.chunkedPostTooLarge"));
}
if (len > 0) {
body.append(buffer, 0, len);
}
}
if (body.getLength() == 0) {
return null;
}
if (body.getLength() < body.getBuffer().length) {
int length = body.getLength();
byte[] result = new byte[length];
System.arraycopy(body.getBuffer(), 0, result, 0, length);
return result;
}
return body.getBuffer();
}
- processParameters()是在Parameters类里面的方法，做的工作就是对请求的数据，做key与value的拆分，然后存放进一个名叫paramHashValues的Map中。后续的request.getParameter取的就是paramHashValues里面的数据
ArrayList<String> values = paramHashValues.get(key);
if (values == null) {
values = new ArrayList<String>(1);
paramHashValues.put(key, values);
}
values.add(value);
解析过程我就不分析了，感兴趣的可以自己查看源码参见tomcat的源码：Parameters类processParameters方法。代码如下：
private void processParameters(byte bytes[], int start, int len, Charset charset) {
if(log.isDebugEnabled()) {
log.debug(sm.getString("parameters.bytes",
new String(bytes, start, len, DEFAULT_CHARSET)));
}
int decodeFailCount = 0;
int pos = start;
int end = start + len;
while(pos < end) {
int nameStart = pos;
int nameEnd = -1;
int valueStart = -1;
int valueEnd = -1;
boolean parsingName = true;
boolean decodeName = false;
boolean decodeValue = false;
boolean parameterComplete = false;
do {
switch(bytes[pos]) {
case '=':
if (parsingName) {
// Name finished. Value starts from next character
nameEnd = pos;
parsingName = false;
valueStart = ++pos;
} else {
// Equals character in value
pos++;
}
break;
case '&':
if (parsingName) {
// Name finished. No value.
nameEnd = pos;
} else {
// Value finished
valueEnd = pos;
}
parameterComplete = true;
pos++;
break;
case '%':
case '+':
// Decoding required
if (parsingName) {
decodeName = true;
} else {
decodeValue = true;
}
pos ++;
break;
default:
pos ++;
break;
}
} while (!parameterComplete && pos < end);
if (pos == end) {
if (nameEnd == -1) {
nameEnd = pos;
} else if (valueStart > -1 && valueEnd == -1){
valueEnd = pos;
}
}
if (log.isDebugEnabled() && valueStart == -1) {
log.debug(sm.getString("parameters.noequal",
Integer.valueOf(nameStart), Integer.valueOf(nameEnd),
new String(bytes, nameStart, nameEnd-nameStart,
DEFAULT_CHARSET)));
}
if (nameEnd <= nameStart ) {
if (valueStart == -1) {
// &&
if (log.isDebugEnabled()) {
log.debug(sm.getString("parameters.emptyChunk"));
}
// Do not flag as error
continue;
}
// &=foo&
UserDataHelper.Mode logMode = userDataLog.getNextMode();
if (logMode != null) {
String extract;
if (valueEnd > nameStart) {
extract = new String(bytes, nameStart, valueEnd
- nameStart, DEFAULT_CHARSET);
} else {
extract = "";
}
String message = sm.getString("parameters.invalidChunk",
Integer.valueOf(nameStart),
Integer.valueOf(valueEnd), extract);
switch (logMode) {
case INFO_THEN_DEBUG:
message += sm.getString("parameters.fallToDebug");
//$FALL-THROUGH$
case INFO:
log.info(message);
break;
case DEBUG:
log.debug(message);
}
}
setParseFailedReason(FailReason.NO_NAME);
continue;
// invalid chunk - it's better to ignore
}
tmpName.setBytes(bytes, nameStart, nameEnd - nameStart);
if (valueStart >= 0) {
tmpValue.setBytes(bytes, valueStart, valueEnd - valueStart);
} else {
tmpValue.setBytes(bytes, 0, 0);
}
// Take copies as if anything goes wrong originals will be
// corrupted. This means original values can be logged.
// For performance - only done for debug
if (log.isDebugEnabled()) {
try {
origName.append(bytes, nameStart, nameEnd - nameStart);
if (valueStart >= 0) {
origValue.append(bytes, valueStart, valueEnd - valueStart);
} else {
origValue.append(bytes, 0, 0);
}
} catch (IOException ioe) {
// Should never happen...
log.error(sm.getString("parameters.copyFail"), ioe);
}
}
try {
String name;
String value;
if (decodeName) {
urlDecode(tmpName);
}
tmpName.setCharset(charset);
name = tmpName.toString();
if (valueStart >= 0) {
if (decodeValue) {
urlDecode(tmpValue);
}
tmpValue.setCharset(charset);
value = tmpValue.toString();
} else {
value = "";
}
try {
addParameter(name, value);
} catch (IllegalStateException ise) {
// Hitting limit stops processing further params but does
// not cause request to fail.
UserDataHelper.Mode logMode = maxParamCountLog.getNextMode();
if (logMode != null) {
String message = ise.getMessage();
switch (logMode) {
case INFO_THEN_DEBUG:
message += sm.getString(
"parameters.maxCountFail.fallToDebug");
//$FALL-THROUGH$
case INFO:
log.info(message);
break;
case DEBUG:
log.debug(message);
}
}
break;
}
} catch (IOException e) {
setParseFailedReason(FailReason.URL_DECODING);
decodeFailCount++;
if (decodeFailCount == 1 || log.isDebugEnabled()) {
if (log.isDebugEnabled()) {
log.debug(sm.getString("parameters.decodeFail.debug",
origName.toString(), origValue.toString()), e);
} else if (log.isInfoEnabled()) {
UserDataHelper.Mode logMode = userDataLog.getNextMode();
if (logMode != null) {
String message = sm.getString(
"parameters.decodeFail.info",
tmpName.toString(), tmpValue.toString());
switch (logMode) {
case INFO_THEN_DEBUG:
message += sm.getString("parameters.fallToDebug");
//$FALL-THROUGH$
case INFO:
log.info(message);
break;
case DEBUG:
log.debug(message);
}
}
}
}
}
tmpName.recycle();
tmpValue.recycle();
// Only recycle copies if we used them
if (log.isDebugEnabled()) {
origName.recycle();
origValue.recycle();
}
}
if (decodeFailCount > 1 && !log.isDebugEnabled()) {
UserDataHelper.Mode logMode = userDataLog.getNextMode();
if (logMode != null) {
String message = sm.getString(
"parameters.multipleDecodingFail",
Integer.valueOf(decodeFailCount));
switch (logMode) {
case INFO_THEN_DEBUG:
message += sm.getString("parameters.fallToDebug");
//$FALL-THROUGH$
case INFO:
log.info(message);
break;
case DEBUG:
log.debug(message);
}
}
}
}
- 由于上述分析的contenttype不为form-data的和x-www-form-urlencoded的不会执行关键二的代码，所以对于请求类型为application/json通过request.getParameter得到的数据为空。
tomcat对请求数据是通过一个个字符判断的，PE的处理方式是通过正则表达式的。关于正则表达式可以参见博客：平安夜快乐
二、request.getAttribute()其实就是RequestFacade类的getAttribute
首先看一下request.setAttribute(name, value)的方法：
public void setAttribute(String name, Object value) {
// Name cannot be null
if (name == null) {
throw new IllegalArgumentException
(sm.getString("coyoteRequest.setAttribute.namenull"));
}
// Null value is the same as removeAttribute()
if (value == null) {
removeAttribute(name);
return;
}
// Special attributes，它的名字一般是tomcat内部的属性，都是心org.apache.catalina开头的。
SpecialAttributeAdapter adapter = specialAttributes.get(name);
if (adapter != null) {
adapter.set(this, name, value);
return;
}
// 安全的检查
...........
Object oldValue = attributes.put(name, value);
// Pass special attributes to the native layer
if (name.startsWith("org.apache.tomcat.")) {
coyoteRequest.setAttribute(name, value);
}
// Notify interested application event listeners
notifyAttributeAssigned(name, value, oldValue);
}
这里的attributes实质上是一个Map，一个多线程安全的Map。至于getAttribute其实就是从Map中得到数据。
private final Map<String, Object> attributes = new ConcurrentHashMap<String, Object>();