一、在应用 java 自带的 HttpClient 时，发现一个景象，即有些以前罕用 http 的 header，是不可能 set 到申请里进行传递的，在 jdk.internal.net.http.common.Utils 类中，能够看到
private static Set<String> getDisallowedHeaders() {Set<String> headers = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
headers.addAll(Set.of("connection", "content-length", "expect", "host", "upgrade"));
String v = getNetProperty("jdk.httpclient.allowRestrictedHeaders");
if (v != null) {
// any headers found are removed from set.
String[] tokens = v.trim().split(",");
for (String token : tokens) {headers.remove(token);
}
return Collections.unmodifiableSet(headers);
} else {return Collections.unmodifiableSet(headers);
}
}connection, content-length, expect, host, upgrade 这几个，属于禁止设置的。因为在 jdk.internal.net.http.HttpRequestBuilderImpl 中，会进行严格的校验
private void checkNameAndValue(String name, String value) {requireNonNull(name, "name");
requireNonNull(value, "value");
if (!isValidName(name)) {throw newIAE("invalid header name: \"%s\"", name);
}
if (!Utils.ALLOWED_HEADERS.test(name, null)) {throw newIAE("restricted header name: \"%s\"", name);
}
if (!isValidValue(value)) {throw newIAE("invalid header value: \"%s\"", value);
}
}二、它本人管制连贯的复用，所以设置不了 connection 这个 header. 从 jdk.internal.net.http.ConnectionPool 中能够看出一些参数
static final long KEEP_ALIVE = Utils.getIntegerNetProperty("jdk.httpclient.keepalive.timeout", 1200); // seconds
static final long MAX_POOL_SIZE = Utils.getIntegerNetProperty("jdk.httpclient.connectionPoolSize", 0); // unbounded从 jdk.internal.net.http.HttpConnection 中能够看到连贯如何放入池中
/**
* Forces a call to the native implementation of the
* connection's channel to verify that this channel is still
* open.
* <p>
* This method should only be called just after an HTTP/1.1
* connection is retrieved from the HTTP/1.1 connection pool.
* It is used to trigger an early detection of the channel state,
* before handling the connection over to the HTTP stack.
* It helps minimizing race conditions where the selector manager
* thread hasn't woken up - or hasn't raised the event, before
* the connection was retrieved from the pool. It helps reduce
* the occurrence of "HTTP/1.1 parser received no bytes"
* exception, when the server closes the connection while
* it's being taken out of the pool.
* <p>
* This method attempts to read one byte from the underlying
* channel. Because the connection was in the pool - there
* should be nothing to read.
* <p>
* If {@code read} manages to read a byte off the connection, this is a
* protocol error: the method closes the connection and returns false.
* If {@code read} returns EOF, the method closes the connection and
* returns false.
* If {@code read} throws an exception, the method returns false.
* Otherwise, {@code read} returns 0, the channel appears to be
* still open, and the method returns true.
* @return true if the channel appears to be still open.
*/
final boolean checkOpen() {if (isOpen()) {
try {
// channel is non blocking
int read = channel().read(ByteBuffer.allocate(1));
if (read == 0) return true;
close();} catch (IOException x) {
debug.log("Pooled connection is no longer operational: %s",
x.toString());
return false;
}
}
return false;
} void closeOrReturnToCache(HttpHeaders hdrs) {if (hdrs == null) {
// the connection was closed by server, eof
Log.logTrace("Cannot return connection to pool: closing {0}", this);
close();
return;
}
HttpClientImpl client = client();
if (client == null) {Log.logTrace("Client released: closing {0}", this);
close();
return;
}
ConnectionPool pool = client.connectionPool();
boolean keepAlive = hdrs.firstValue("Connection")
.map((s) -> !s.equalsIgnoreCase("close"))
.orElse(true);
if (keepAlive && checkOpen()) {Log.logTrace("Returning connection to the pool: {0}", this);
pool.returnToPool(this);
} else {Log.logTrace("Closing connection (keepAlive={0}, isOpen={1}): {2}",
keepAlive, isOpen(), this);
close();}
}