登录操作一般都是我们触发的：
Subject subject = SecurityUtils.getSubject();
AuthenticationToken authenticationToken = new ...
subject.login(authenticationToken);Subject的登录将委托给SecurityManager，SecurityManager的login方法实际上是产生了一个新的Subject，然后将相关属性赋予当前调用者Subject:
public void login(AuthenticationToken token) throws AuthenticationException {
clearRunAsIdentitiesInternal();
Subject subject = securityManager.login(this, token);
PrincipalCollection principals;
String host = null;
if (subject instanceof DelegatingSubject) {
DelegatingSubject delegating = (DelegatingSubject) subject;
//we have to do this in case there are assumed identities - we don't want to lose the 'real' principals:
principals = delegating.principals;
host = delegating.host;
} else {
principals = subject.getPrincipals();
}
if (principals == null || principals.isEmpty()) {
String msg = "Principals returned from securityManager.login( token ) returned a null or " +
"empty value. This value must be non null and populated with one or more elements.";
throw new IllegalStateException(msg);
}
this.principals = principals;
this.authenticated = true;
if (token instanceof HostAuthenticationToken) {
host = ((HostAuthenticationToken) token).getHost();
}
if (host != null) {
this.host = host;
}
Session session = subject.getSession(false);
if (session != null) {
this.session = decorate(session);
} else {
this.session = null;
}
}DefaultSecurityManager实现了login方法：
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;
}
父类AuthenticatingSecurityManager实现authenticate方法：
public abstract class AuthenticatingSecurityManager extends RealmSecurityManager {
private Authenticator authenticator;
public AuthenticatingSecurityManager() {
super();
this.authenticator = new ModularRealmAuthenticator();
}
public AuthenticationInfo authenticate(AuthenticationToken token) throws AuthenticationException {
return this.authenticator.authenticate(token);
}
//......
}利用一个ModularRealmAuthenticator类型的authenticator来实现：
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);
}
}然后根据realms集合是单个还是多个分别处理，最终无非是这样：
AuthenticationInfo info = realm.getAuthenticationInfo(token);
父类AuthenticatingRealm的getAuthenticationInfo方法实现了info的获取和身份的校验，仅仅调用自己实现的realm的doGetAuthenticationInfo方法，初步验证后构造一个SimpleAuthenticationInfo：
SimplePrincipalCollection principalCollection = new SimplePrincipalCollection(principals, this.getName());
return new SimpleAuthenticationInfo(principalCollection, authenticationInfo.getCredentials());AuthenticatingRealm的getAuthenticationInfo方法逻辑如下：
首先去缓存找info:
AuthenticationInfo info = getCachedAuthenticationInfo(token);
缓存没有则调用子类实现的方法：
info = doGetAuthenticationInfo(token);
info不为null的时候就要验证了（这里还可以加密验证）：
assertCredentialsMatch(token, info);
两次创建Subject
进入AbstractShiroFilter的时候，会默认创建一个Subject，这个是在Subject接口中的内部类实现的，但是同样也是调用了DefaultSecurityManager中的createSubject方法：
public Subject createSubject(SubjectContext subjectContext) {
//create a copy so we don't modify the argument's backing map:
SubjectContext context = copy(subjectContext);
//ensure that the context has a SecurityManager instance, and if not, add one:
context = ensureSecurityManager(context);
//Resolve an associated Session (usually based on a referenced session ID), and place it in the context before
//sending to the SubjectFactory. The SubjectFactory should not need to know how to acquire sessions as the
//process is often environment specific - better to shield the SF from these details:
context = resolveSession(context);
//Similarly, the SubjectFactory should not require any concept of RememberMe - translate that here first
//if possible before handing off to the SubjectFactory:
context = resolvePrincipals(context);
Subject subject = doCreateSubject(context);
//save this subject for future reference if necessary:
//(this is needed here in case rememberMe principals were resolved and they need to be stored in the
//session, so we don't constantly rehydrate the rememberMe PrincipalCollection on every operation).
//Added in 1.2:
save(subject);
return subject;
}初次进入shiroFilter，会创建一个Subject，这个Subject没有验证通过，保留了三个属性：request，response，securityManager。
当我们调用subject.login的时候，我们在DefaultSecurityManager中为subjectContext设置了相关属性：
protected Subject createSubject(AuthenticationToken token, AuthenticationInfo info, Subject existing) {
SubjectContext context = createSubjectContext();
context.setAuthenticated(true);
context.setAuthenticationToken(token);
context.setAuthenticationInfo(info);
if (existing != null) {
context.setSubject(existing);
}
// 这个方法的具体实现在上面
return createSubject(context);
}一个save方法为我们构造了session：
save(subject);
这个session是根据我们的principals（放在登录成功返回的那个AuthenticationInfo中）构造的。
下一次进入的时候，依然是：
final Subject subject = createSubject(request, response);
但是下面的方法为我们找回了session，通过request.getSession(false)就可以取到。
context = resolveSession(context);
有了session后其他的东西都可以恢复，这样就可以识别并维持一个subject的状态，即使每次都重新创建了Subject对象。
具体是在DefaultWebSubjectFactory这个方法里恢复的，并且通过构造器赋值给下一个新的subject了：
public Subject createSubject(SubjectContext context) {
if (!(context instanceof WebSubjectContext)) {
return super.createSubject(context);
}
WebSubjectContext wsc = (WebSubjectContext) context;
SecurityManager securityManager = wsc.resolveSecurityManager();
Session session = wsc.resolveSession();
boolean sessionEnabled = wsc.isSessionCreationEnabled();
PrincipalCollection principals = wsc.resolvePrincipals();
boolean authenticated = wsc.resolveAuthenticated();
String host = wsc.resolveHost();
ServletRequest request = wsc.resolveServletRequest();
ServletResponse response = wsc.resolveServletResponse();
return new WebDelegatingSubject(principals, authenticated, host, session, sessionEnabled,
request, response, securityManager);
}举两个例子：
PrincipalCollection principals = wsc.resolvePrincipals();
-> principals = (PrincipalCollection) session.getAttribute(PRINCIPALS_SESSION_KEY);
boolean authenticated = wsc.resolveAuthenticated();
-> Session session = resolveSession();
if (session != null) {
Boolean sessionAuthc = (Boolean) session.getAttribute(AUTHENTICATED_SESSION_KEY);
authc = sessionAuthc != null && sessionAuthc;
}通过上面两个方法就恢复了Subject的两个属性，其实都是存放在session中。
其实只要你使用了Shiro，不管你是否登录，核心过滤器都会为我们构造Subject实例，当我们主动调用subject.login方法时，会间接调用我们自己实现的realm的doGetAuthenticationInfo，根据我们在数据库中获取的信息（存放在info中）和调用login方法时传递的AuthenticationToken中的信息对比。
当info为null或者抛出了AuthenticationException异常，都视为登录失败。