Apache HttpComponents学习笔记（三）：配置HttpClient

释放系统资源

只有关闭streamed Entity的content stream或/和关闭响应对象，才能释放系统资源。区别在于，关闭content stream能保持底层连接的存活，而关闭响应对象会立即关闭并丢弃底层连接。EntityUtils.consume(HttpEntity)方法能用来保证streamed Entity能正常关闭content stream.

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);
try {
HttpEntity entity = response.getEntity();
if (entity != null) {
InputStream instream = entity.getContent();
try {
// do something useful
} finally {
instream.close();
}
}
} finally {
response.close();
}

如果只关闭响应对象，不关闭streamed Entity的content stream, 也能正常释放系统资源，并且无法再从content stream中读取内容。这适用于只需要从content stream中读取部分内容就释放资源的场景。
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);
try {
HttpEntity entity = response.getEntity();
if (entity != null) {
InputStream instream = entity.getContent();
int byteOne = instream.read();
int byteTwo = instream.read();
// Do not need the rest
}
} finally {
response.close();
}

EntityUtils的toByteArray和toString方法可以用于从Entity里读取完整的内容到byte数组或字符串里。但是，不建议这么做，除非Entity来自可信的服务器并且内容长度有限。
CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);
try {
HttpEntity entity = response.getEntity();
if (entity != null) {
long len = entity.getContentLength();
if (len != -1 && len < 2048) {
System.out.println(EntityUtils.toString(entity));
} else {
// Stream content out
}
}
} finally {
response.close();
}

使用ResponseHandler

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/json");

ResponseHandler<MyJsonObject> rh = new ResponseHandler<MyJsonObject>() {

@Override
public JsonObject handleResponse(
final HttpResponse response) throws IOException {
StatusLine statusLine = response.getStatusLine();
HttpEntity entity = response.getEntity();
if (statusLine.getStatusCode() >= 300) {
throw new HttpResponseException(
statusLine.getStatusCode(),
statusLine.getReasonPhrase());
}
if (entity == null) {
throw new ClientProtocolException("Response contains no content");
}
Gson gson = new GsonBuilder().create();
ContentType contentType = ContentType.getOrDefault(entity);
Charset charset = contentType.getCharset();
Reader reader = new InputStreamReader(entity.getContent(), charset);
return gson.fromJson(reader, MyJsonObject.class);
}
};
MyJsonObject myjson = client.execute(httpget, rh);

配置Keep-Alive timeout

ConnectionKeepAliveStrategy keepAliveStrat = new DefaultConnectionKeepAliveStrategy() {

@Override
public long getKeepAliveDuration(
HttpResponse response,
HttpContext context) {
long keepAlive = super.getKeepAliveDuration(response, context);
if (keepAlive == -1) {
// Keep connections alive 5 seconds if a keep-alive value
// has not be explicitly set by the server
keepAlive = 5000;
}
return keepAlive;
}

};
CloseableHttpClient httpclient = HttpClients.custom()
.setKeepAliveStrategy(keepAliveStrat)
.build();

配置HTTP protocol interceptors

CloseableHttpClient httpclient = HttpClients.custom()
.addInterceptorLast(new HttpRequestInterceptor() {

public void process(
final HttpRequest request,
final HttpContext context) throws HttpException, IOException {
AtomicInteger count = (AtomicInteger) context.getAttribute("count");
request.addHeader("Count", Integer.toString(count.getAndIncrement()));
}

})
.build();

AtomicInteger count = new AtomicInteger(1);
HttpClientContext localContext = HttpClientContext.create();
localContext.setAttribute("count", count);

HttpGet httpget = new HttpGet("http://localhost/");
for (int i = 0; i < 10; i++) {
CloseableHttpResponse response = httpclient.execute(httpget, localContext);
try {
HttpEntity entity = response.getEntity();
} finally {
response.close();
}
}

配置请求重试Handler

HttpRequestRetryHandler myRetryHandler = new HttpRequestRetryHandler() {

public boolean retryRequest(
IOException exception,
int executionCount,
HttpContext context) {
if (executionCount >= 5) {
// Do not retry if over max retry count
return false;
}
if (exception instanceof InterruptedIOException) {
// Timeout
return false;
}
if (exception instanceof UnknownHostException) {
// Unknown host
return false;
}
if (exception instanceof ConnectTimeoutException) {
// Connection refused
return false;
}
if (exception instanceof SSLException) {
// SSL handshake exception
return false;
}
HttpClientContext clientContext = HttpClientContext.adapt(context);
HttpRequest request = clientContext.getRequest();
boolean idempotent = !(request instanceof HttpEntityEnclosingRequest);
if (idempotent) {
// Retry if the request is considered idempotent
return true;
}
return false;
}

};
CloseableHttpClient httpclient = HttpClients.custom()
.setRetryHandler(myRetryHandler)
.build();

配置重导向策略

LaxRedirectStrategy redirectStrategy = new LaxRedirectStrategy();
CloseableHttpClient httpclient = HttpClients.custom()
.setRedirectStrategy(redirectStrategy)
.build();