详解shrio的认证（登录）过程

shrio是一个比较轻量级的安全框架，主要的作用是在后端承担认证和授权的工作。今天就讲一下shrio进行认证的一个过程。
首先先介绍一下在认证过程中的几个关键的对象：

• Subject：主体

访问系统的用户，主体可以是用户、程序等，进行认证的都称为主体；

• Principal：身份信息

是主体（subject）进行身份认证的标识，标识必须具有唯一性，如用户名、手机号、邮箱地址等，一个主体可以有多个身份，但是必须有一个主身份（Primary Principal）。

• credential：凭证信息

是只有主体自己知道的安全信息，如密码、证书等。
接着我们就进入认证的具体过程：
首先是从前端的登录表单中接收到用户输入的token（username + password）：

@RequestMapping("/login")
public String login(@RequestBody Map user){
  Subject subject = SecurityUtils.getSubject();
  UsernamePasswordToken usernamePasswordToken = new UsernamePasswordToken(user.get("email").toString(), user.get("password").toString());
   try {
      subject.login(usernamePasswordToken);
   } catch (UnknownAccountException e) {
      return "邮箱不存在！";
   } catch (AuthenticationException e) {
      return "账号或密码错误！";
   }
    return "登录成功！";
  }

这里的usernamePasswordToken（以下简称token）就是用户名和密码的一个结合对象，然后调用subject的login方法将token传入开始认证过程。
接着会发现subject的login方法调用的其实是securityManager的login方法：

Subject subject = securityManager.login(this, token);

再往下看securityManager的login方法内部：

public Subject login(Subject subject, AuthenticationToken token) throws AuthenticationException {
  AuthenticationInfo info;
   try {
      info = authenticate(token);
   } catch (AuthenticationException ae) {
      try {
        onFailedLogin(token, ae, subject);
   } catch (Exception e) {
        if (log.isInfoEnabled()) {
          log.info("onFailedLogin method threw an " +
              "exception. Logging and propagating original AuthenticationException.", e);
   }
      }
      throw ae; //propagate
   }
    Subject loggedIn = createSubject(token, info, subject);
   onSuccessfulLogin(token, info, loggedIn);
   return loggedIn;
}

上面代码的关键在于：

info = authenticate(token);

即将token传入authenticate方法中得到一个AuthenticationInfo类型的认证信息。
以下是authenticate方法的具体内容：

public final AuthenticationInfo authenticate(AuthenticationToken token) throws AuthenticationException {
  if (token == null) {
    throw new IllegalArgumentException("Method argument (authentication token) cannot be null.");
  }
  log.trace("Authentication attempt received for token [{}]", token);
  AuthenticationInfo info;
  try {
    info = doAuthenticate(token);
  if (info == null) {
      String msg = "No account information found for authentication token [" + token + "] by this " +
          "Authenticator instance. Please check that it is configured correctly.";
  throw new AuthenticationException(msg);
  }
  } catch (Throwable t) {
    AuthenticationException ae = null;
  if (t instanceof AuthenticationException) {
      ae = (AuthenticationException) t;
  }
    if (ae == null) {
      //Exception thrown was not an expected AuthenticationException. Therefore it is probably a little more
  //severe or unexpected. So, wrap in an AuthenticationException, log to warn, and propagate: String msg = "Authentication failed for token submission [" + token + "]. Possible unexpected " +
          "error? (Typical or expected login exceptions should extend from AuthenticationException).";
  ae = new AuthenticationException(msg, t);
  if (log.isWarnEnabled())
        log.warn(msg, t);
  }
    try {
      notifyFailure(token, ae);
  } catch (Throwable t2) {
      if (log.isWarnEnabled()) {
        String msg = "Unable to send notification for failed authentication attempt - listener error?. " +
            "Please check your AuthenticationListener implementation(s). Logging sending exception " +
            "and propagating original AuthenticationException instead...";
  log.warn(msg, t2);
  }
    }
    throw ae;
  }
  log.debug("Authentication successful for token [{}]. Returned account [{}]", token, info);
  notifySuccess(token, info);
  return info;
}

首先就是判断token是否为空，不为空再将token传入doAuthenticate方法中：

protected AuthenticationInfo doAuthenticate(AuthenticationToken authenticationToken) throws AuthenticationException {
  assertRealmsConfigured();
  Collection<Realm> realms = getRealms();
  if (realms.size() == 1) {
    return doSingleRealmAuthentication(realms.iterator().next(), authenticationToken);
  } else {
    return doMultiRealmAuthentication(realms, authenticationToken);
  }
}

这一步是判断是有单个Reaml验证还是多个Reaml验证，单个就执行doSingleRealmAuthentication()方法，多个就执行doMultiRealmAuthentication()方法。
一般情况下是单个验证：

protected AuthenticationInfo doSingleRealmAuthentication(Realm realm, AuthenticationToken token) {
  if (!realm.supports(token)) {
    String msg = "Realm [" + realm + "] does not support authentication token [" +
        token + "]. Please ensure that the appropriate Realm implementation is " +
        "configured correctly or that the realm accepts AuthenticationTokens of this type.";
    throw new UnsupportedTokenException(msg);
  }
  AuthenticationInfo info = realm.getAuthenticationInfo(token);
  if (info == null) {
    String msg = "Realm [" + realm + "] was unable to find account data for the " +
        "submitted AuthenticationToken [" + token + "].";
    throw new UnknownAccountException(msg);
  }
  return info;
}

这一步中首先判断是否支持Realm，只有支持Realm才调用realm.getAuthenticationInfo(token)获取info。

public final AuthenticationInfo getAuthenticationInfo(AuthenticationToken token) throws AuthenticationException {
  AuthenticationInfo info = getCachedAuthenticationInfo(token);
  if (info == null) {
    //otherwise not cached, perform the lookup:
    info = doGetAuthenticationInfo(token);
    log.debug("Looked up AuthenticationInfo [{}] from doGetAuthenticationInfo", info);
    if (token != null && info != null) {
      cacheAuthenticationInfoIfPossible(token, info);
    }
  } else {
    log.debug("Using cached authentication info [{}] to perform credentials matching.", info);
  }
  if (info != null) {
    assertCredentialsMatch(token, info);
  } else {
    log.debug("No AuthenticationInfo found for submitted AuthenticationToken [{}]. Returning null.", token);
  }
  return info;
}

首先查看Cache中是否有该token的info，如果有，则直接从Cache中去即可。如果是第一次登录，则Cache中不会有该token的info，需要调用doGetAuthenticationInfo(token)方法获取，并将结果加入到Cache中，方便下次使用。而这里调用的doGetAuthenticationInfo()方法就是我们在自己重写的方法，具体的内容是自定义了对拿到的这个token的一个处理的过程：

protected AuthenticationInfo doGetAuthenticationInfo(AuthenticationToken authenticationToken) throws AuthenticationException {
  if (authenticationToken.getPrincipal() == null)
    return null;
  String email = authenticationToken.getPrincipal().toString();
  User user = userService.findByEmail(email);
  if (user == null)
    return null;
  else return new SimpleAuthenticationInfo(email, user.getPassword(), getName());
}

这其中进行了几步判断：首先是判断传入的用户名是否为空，在判断传入的用户名在本地的数据库中是否存在，不存在则返回一个用户名不存在的Exception。以上两部通过之后生成一个包括传入用户名和密码的info，注意此时关于用户名的验证已经完成，接下来进入对密码的验证。
将这一步得到的info返回给getAuthenticationInfo方法中的

assertCredentialsMatch(token, info);

此时的info是正确的用户名和密码的信息，token是输入的用户名和密码的信息，经过前面步骤的验证过程，用户名此时已经是真是存在的了，这一步就是验证输入的用户名和密码的对应关系是否正确。

protected void assertCredentialsMatch(AuthenticationToken token, AuthenticationInfo info) throws AuthenticationException {
  CredentialsMatcher cm = getCredentialsMatcher();
  if (cm != null) {
    if (!cm.doCredentialsMatch(token, info)) {
      //not successful - throw an exception to indicate this:
      String msg = "Submitted credentials for token [" + token + "] did not match the expected credentials.";
      throw new IncorrectCredentialsException(msg);
    }
  }
  else {
    throw new AuthenticationException("A CredentialsMatcher must be configured in order to verify " +
        "credentials during authentication. If you do not wish for credentials to be examined, you " +
        "can configure an " + AllowAllCredentialsMatcher.class.getName() + " instance.");
  }
}

上面步骤就是验证token中的密码的和info中的密码是否对应的代码。这一步验证完成之后，整个shrio认证的过程就结束了。

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