HttpClient的认证机制，并给出示例代码。

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HttpClient三种不同的认证方案: Basic, Digest and NTLM. 这些方案可用于服务器或代理对客户端的认证，简称服务器认证或代理认证。

1．服务器认证(Server Authentication)

HttpClient处理服务器认证几乎是透明的，仅需要开发人员提供登录信息(login credentials)。登录信息保存在HttpState类的实例中，可以通过 setCredentials(String realm, Credentials cred)和getCredentials(String realm)来获取或设置。

HttpClient内建的自动认证，可以通过HttpMethod类的setDoAuthentication(boolean doAuthentication)方法关闭，而且这次关闭只影响HttpMethod当前的实例。

1.1抢先认证(Preemptive Authentication)

在这种模式时，HttpClient会主动将basic认证应答信息传给服务器，即使在某种情况下服务器可能返回认证失败的应答，这样做主要是为了减少连接的建立。使用该机制如下所示：

client.getParams().setAuthenticationPreemptive(true);

抢先认证模式也提供对于特定目标或代理的缺省认证。如果没有提供缺省的认证信息，则该模式会失效。

Credentials defaultcreds = new UsernamePasswordCredentials("username", "password");

client.getState().setCredentials(new AuthScope("myhost", 80, AuthScope.ANY_REALM), defaultcreds);

Httpclient实现的抢先认证遵循rfc2617.

A client SHOULD assume that all paths at or deeper than the depth of the last symbolic element in the path field of the Request-URI also are within the protection space specified by the Basic realm value of the current challenge. A client MAY preemptively send the corresponding Authorization header with requests for resources in that space without receipt of another challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the Proxy-Authorization header field without receiving another challenge from the proxy server.

1.2服务器认证的安全方面考虑

当需要与不被信任的站点或web应用通信时，应该谨慎使用缺省的认证机制。当启动(activate)抢先认证模式，或者认证中没有明确给出认证域，主机的HttpClient将使用缺省的认证机制去试图获得目标站点的授权。

如果你提供的认证信息是敏感的，你应该指定认证域。不推荐将认证域指定为AuthScope.ANY。（只有在debugging情况下，才使用）

// To be avoided unless in debug mode

Credentials defaultcreds = new UsernamePasswordCredentials("username", "password");

client.getState().setCredentials(AuthScope.ANY, defaultcreds);

2.代理认证(proxy authentication)

　　除了登录信息需单独存放以外，代理认证与服务器认证几乎一致。用 setProxyCredentials(String realm, Credentials cred)和getProxyCredentials(String realm)设、取登录信息。

3.认证方案(authentication schemes)

3.1Basic

是HTTP中规定最早的也是最兼容的方案，遗憾的是也是最不安全的一个方案，因为它以明码传送用户名和密码。它要求一个UsernamePasswordCredentials实例，可以指定服务器端的访问空间或采用默认的登录信息。

3.2 Digest

是在HTTP1.1 中增加的一个方案，虽然不如Basic得到的软件支持多，但还是有广泛的使用。Digest方案比Basic方案安全得多，因它根本就不通过网络传送实际的密码，传送的是利用这个密码对从服务器传来的一个随机数(nonce)的加密串。

它要求一个UsernamePasswordCredentials实例，可以指定服务器端的访问空间或采用默认的登录信息。

3.3 NTLM

这是HttpClient支持的最复杂的认证协议。它Microsoft设计的一个私有协议，没有公开的规范说明。一开始由于设计的缺陷，NTLM的安全性比 Digest差，后来经过一个ServicePack补丁后，安全性则比较Digest高。

NTLM需要一个NTCredentials实例。注意，由于NTLM不使用访问空间(realms)的概念，HttpClient利用服务器的域名作访问空间的名字。还需要注意，提供给 NTCredentials的用户名，不要用域名的前缀 - 如: "adrian" 是正确的，而 "DOMAIN\adrian" 则是错的。

NTLM认证的工作机制与basic和digest有很大的差别。这些差别一般由HttpClient处理，但理解这些差别有助避免在使用NTLM认证时出现错误。

[1] 从HttpClientAPI的角度来看，NTLM与其它认证方式一样的工作，差别是需要提供'NTCredentials'实例而不是'UsernamePasswordCredentials'(其实，前者只是扩展了后者)

[2] 对NTLM认证，访问空间是连接到的机器的域名，这对多域名主机会有一些麻烦。只有HttpClient连接中指定的域名才是认证用的域名。建议将realm设为null以使用默认的设置。

[3] NTLM只是认证了一个连接而不是一请求，所以每当一个新的连接建立就要进行一次认证，且在认证的过程中保持连接是非常重要的。因此，NTLM不能同时用于代理认证和服务器认证，也不能用于HTTP1.0连接或服务器不支持持久连接(keep-alives)的情况。

关于NTLM认证机制更详细的研究，可参考http://davenport.sourceforge.net/ntlm.html 。

3.4选择认证

一些服务器支持多种认证方案。假设一次只能使用一种认证方案，HttpClient必须选择使用哪种。HttpClient选择是基于NTLM, Digest, Basic顺序的。

在具体情况下，可以更改该顺序。可通过参数'http.auth.scheme-priority'来实现，该参数值应该被存放在一个String类型的List中。选择优先级是按插入顺序确定的。

HttpClient client = new HttpClient();

List authPrefs = new ArrayList(2);

authPrefs.add(AuthPolicy.DIGEST);

authPrefs.add(AuthPolicy.BASIC);

// This will exclude the NTLM authentication scheme

client.getParams().setParameter(AuthPolicy.AUTH_SCHEME_PRIORITY, authPrefs);

3.5定制认证方案

HttpClient本身支持basic, digest, and NTLM这三种认证方案。同时，它也提供了加载额外定制的认证方案的功能（通过AuthScheme接口实现）。需要使用定制的认证方案，必须实现下面的步骤：

[1]实现AuthScheme接口。

[2]通过AuthPolicy.registerAuthScheme() 注册定制的AuthScheme。

[3]将定制的AuthScheme加入到AuthPolicy.AUTH_SCHEME_PRIORITY中。

4．示例

4.1Basic authentication

Java代码

import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.UsernamePasswordCredentials;
import org.apache.commons.httpclient.auth.AuthScope;
import org.apache.commons.httpclient.methods.GetMethod;
/**
* A simple example that uses HttpClient to perform a GET using Basic
* Authentication. Can be run standalone without parameters.
*
* You need to have JSSE on your classpath for JDK prior to 1.4
*
* @author Michael Becke
*/
public class BasicAuthenticationExample {
/**
* Constructor for BasicAuthenticatonExample.
*/
public BasicAuthenticationExample() {
super();
}
public static void main(String[] args) throws Exception {
HttpClient client = new HttpClient();
// pass our credentials to HttpClient, they will only be used for
// authenticating to servers with realm "realm" on the host
// "www.verisign.com", to authenticate against
// an arbitrary realm or host change the appropriate argument to null.
client.getState().setCredentials(
new AuthScope("www.verisign.com", 443, "realm"),
new UsernamePasswordCredentials("username", "password"));
// create a GET method that reads a file over HTTPS, we're assuming
// that this file requires basic authentication using the realm above.
GetMethod get = new GetMethod(
"https://www.verisign.com/products/index.html");
// Tell the GET method to automatically handle authentication. The
// method will use any appropriate credentials to handle basic
// authentication requests. Setting this value to false will cause
// any request for authentication to return with a status of 401.
// It will then be up to the client to handle the authentication.
get.setDoAuthentication(true);
try {
// execute the GET
int status = client.executeMethod(get);
// print the status and response
System.out.println(status + "\n" + get.getResponseBodyAsString());
} finally {
// release any connection resources used by the method
get.releaseConnection();
}
}
}

4.2 Alternate authentication

Java代码

import java.util.ArrayList;
import java.util.List;
import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.UsernamePasswordCredentials;
import org.apache.commons.httpclient.auth.AuthPolicy;
import org.apache.commons.httpclient.auth.AuthScope;
import org.apache.commons.httpclient.methods.GetMethod;
/**
* <p>
* A simple example that uses alternate authentication scheme selection if
* several authentication challenges are returned.
* </p>
*
* <p>
* Per default HttpClient picks the authentication challenge in the following
* order of preference: NTLM, Digest, Basic. In certain cases it may be
* desirable to force the use of a weaker authentication scheme.
* </p>
*
* @author Oleg Kalnichevski
*/
public class AlternateAuthenticationExample {
/**
* Constructor for BasicAuthenticatonExample.
*/
public AlternateAuthenticationExample() {
super();
}
public static void main(String[] args) throws Exception {
HttpClient client = new HttpClient();
client.getState().setCredentials(
new AuthScope("myhost", 80, "myrealm"),
new UsernamePasswordCredentials("username", "password"));
// Suppose the site supports several authetication schemes: NTLM and
// Basic
// Basic authetication is considered inherently insecure. Hence, NTLM
// authentication
// is used per default
// This is to make HttpClient pick the Basic authentication scheme over
// NTLM & Digest
List<String> authPrefs = new ArrayList<String>(3);
authPrefs.add(AuthPolicy.BASIC);
authPrefs.add(AuthPolicy.NTLM);
authPrefs.add(AuthPolicy.DIGEST);
client.getParams().setParameter(AuthPolicy.AUTH_SCHEME_PRIORITY,
authPrefs);
GetMethod httpget = new GetMethod(
"http://myhost/protected/auth-required.html");
try {
int status = client.executeMethod(httpget);
// print the status and response
System.out.println(status);
System.out.println(httpget.getStatusLine());
System.out.println(httpget.getResponseBodyAsString());
} finally {
// release any connection resources used by the method
httpget.releaseConnection();
}
}
}

4.3 Custom authentication

Java代码

import java.util.ArrayList;
import java.util.Collection;
import org.apache.commons.httpclient.Credentials;
import org.apache.commons.httpclient.HttpMethod;
import org.apache.commons.httpclient.auth.AuthPolicy;
import org.apache.commons.httpclient.auth.AuthScheme;
import org.apache.commons.httpclient.auth.AuthenticationException;
import org.apache.commons.httpclient.auth.MalformedChallengeException;
import org.apache.commons.httpclient.params.DefaultHttpParams;
import org.apache.commons.httpclient.params.HttpParams;
/**
* A simple custom AuthScheme example. The included auth scheme is meant for
* demonstration purposes only. It does not actually implement a usable
* authentication method.
*/
public class CustomAuthenticationExample {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
// register the auth scheme
AuthPolicy.registerAuthScheme(SecretAuthScheme.NAME,
SecretAuthScheme.class);
// include the scheme in the AuthPolicy.AUTH_SCHEME_PRIORITY preference,
// this can be done on a per-client or per-method basis but we'll do it
// globally for this example
HttpParams params = DefaultHttpParams.getDefaultParams();
ArrayList<String> schemes = new ArrayList<String>();
schemes.add(SecretAuthScheme.NAME);
schemes.addAll((Collection) params
.getParameter(AuthPolicy.AUTH_SCHEME_PRIORITY));
params.setParameter(AuthPolicy.AUTH_SCHEME_PRIORITY, schemes);
// now that our scheme has been registered we can execute methods
// against
// servers that require "Secret" authentication...
}
/**
* A custom auth scheme that just uses "Open Sesame" as the authentication
* string.
*/
private class SecretAuthScheme implements AuthScheme {
public static final String NAME = "Secret";
public SecretAuthScheme() {
// All auth schemes must have a no arg constructor.
}
public String authenticate(Credentials credentials, HttpMethod method)
throws AuthenticationException {
return "Open Sesame";
}
public String authenticate(Credentials credentials, String method,
String uri) throws AuthenticationException {
return "Open Sesame";
}
public String getID() {
return NAME;
}
public String getParameter(String name) {
// this scheme does not use parameters, see RFC2617Scheme for an
// example
return null;
}
public String getRealm() {
// this scheme does not use realms
return null;
}
public String getSchemeName() {
return NAME;
}
public boolean isConnectionBased() {
return false;
}
public void processChallenge(String challenge)
throws MalformedChallengeException {
// Nothing to do here, this is not a challenge based
// auth scheme. See NTLMScheme for a good example.
}
public boolean isComplete() {
// again we're not a challenge based scheme so this is always true
return true;
}
}
}

4.4 Interactive authentication

Java代码

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import org.apache.commons.httpclient.Credentials;
import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.NTCredentials;
import org.apache.commons.httpclient.UsernamePasswordCredentials;
import org.apache.commons.httpclient.auth.AuthScheme;
import org.apache.commons.httpclient.auth.CredentialsNotAvailableException;
import org.apache.commons.httpclient.auth.CredentialsProvider;
import org.apache.commons.httpclient.auth.NTLMScheme;
import org.apache.commons.httpclient.auth.RFC2617Scheme;
import org.apache.commons.httpclient.methods.GetMethod;
/**
* A simple example that uses HttpClient to perform interactive authentication.
*
* @author Oleg Kalnichevski
*/
public class InteractiveAuthenticationExample {
/**
* Constructor for InteractiveAuthenticationExample.
*/
public InteractiveAuthenticationExample() {
super();
}
public static void main(String[] args) throws Exception {
InteractiveAuthenticationExample demo = new InteractiveAuthenticationExample();
demo.doDemo();
}
private void doDemo() throws IOException {
HttpClient client = new HttpClient();
client.getParams().setParameter(CredentialsProvider.PROVIDER,
new ConsoleAuthPrompter());
GetMethod httpget = new GetMethod(
"http://target-host/requires-auth.html");
httpget.setDoAuthentication(true);
try {
// execute the GET
int status = client.executeMethod(httpget);
// print the status and response
System.out.println(status);
System.out.println(httpget.getStatusLine().toString());
System.out.println(httpget.getResponseBodyAsString());
} finally {
// release any connection resources used by the method
httpget.releaseConnection();
}
}
public class ConsoleAuthPrompter implements CredentialsProvider {
private BufferedReader in = null;
public ConsoleAuthPrompter() {
super();
this.in = new BufferedReader(new InputStreamReader(System.in));
}
private String readConsole() throws IOException {
return this.in.readLine();
}
public Credentials getCredentials(final AuthScheme authscheme,
final String host, int port, boolean proxy)
throws CredentialsNotAvailableException {
if (authscheme == null) {
return null;
}
try {
if (authscheme instanceof NTLMScheme) {
System.out.println(host + ":" + port
+ " requires Windows authentication");
System.out.print("Enter domain: ");
String domain = readConsole();
System.out.print("Enter username: ");
String user = readConsole();
System.out.print("Enter password: ");
String password = readConsole();
return new NTCredentials(user, password, host, domain);
} else if (authscheme instanceof RFC2617Scheme) {
System.out.println(host + ":" + port
+ " requires authentication with the realm '"
+ authscheme.getRealm() + "'");
System.out.print("Enter username: ");
String user = readConsole();
System.out.print("Enter password: ");
String password = readConsole();
return new UsernamePasswordCredentials(user, password);
} else {
throw new CredentialsNotAvailableException(
"Unsupported authentication scheme: "
+ authscheme.getSchemeName());
}
} catch (IOException e) {
throw new CredentialsNotAvailableException(e.getMessage(), e);
}
}
}
}

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