后端：用户登录、退出登录、获取用户状态相关功能完成；引入SessionManager；数据库：user_identity改为group；前端：添加登录页面、后台管理页面框架，axios POST请求Content-Type问题修复，引入md5、sha1 js库；小问题调整

bookshelfplus/src/main/java/plus/bookshelf/Common/SessionManager/LocalSessionManager.java

@@ -0,0 +1,48 @@
+package plus.bookshelf.Common.SessionManager;
+
+import javax.servlet.http.HttpServletRequest;
+
+public class LocalSessionManager implements SessionManager {
+
+    /**
+     * 私有化构造函数
+     */
+    private LocalSessionManager(HttpServletRequest httpServletRequest) {
+        this.httpServletRequest = httpServletRequest;
+    }
+
+    static SessionManager sessionManager = null;
+    // static SessionManager sessionManager = new LocalSessionManager();
+
+    /**
+     * 通过此方法获取当前类的实例
+     *
+     * @return
+     */
+    public static SessionManager getInstance(HttpServletRequest httpServletRequest) {
+        if (sessionManager == null)
+            sessionManager = new LocalSessionManager(httpServletRequest);
+        return sessionManager;
+    }
+
+    HttpServletRequest httpServletRequest;
+
+    @Override
+    public Object getValue(String key) {
+        try {
+            return httpServletRequest.getSession().getAttribute(key);
+        } catch (Exception e) {
+            return null;
+        }
+    }
+
+    @Override
+    public void setValue(String key, Object value) {
+        httpServletRequest.getSession().setAttribute(key, value);
+    }
+
+    @Override
+    public void remove(String key) {
+        httpServletRequest.getSession().removeAttribute(key);
+    }
+}

bookshelfplus/src/main/java/plus/bookshelf/Common/SessionManager/SessionManager.java

@@ -0,0 +1,23 @@
+package plus.bookshelf.Common.SessionManager;
+
+public abstract interface SessionManager {
+    /**
+     * 获取 Session
+     * @param key
+     * @return
+     */
+    Object getValue(String key);
+
+    /**
+     * 设置 Session
+     * @param key
+     * @param value
+     */
+    void setValue(String key, Object value);
+
+    /**
+     * 移除 Session
+     * @param key
+     */
+    void remove(String key);
+}