问题描述：在Angular Universal / Node应用中，后端无法访问会话，并且每次重新加载页面时都会创建一个新的会话。

解决方法：要解决此问题，可以使用Angular Universal的TransferState模块来将会话状态从客户端传递到服务器端。下面是一个解决方法的代码示例：

1. 在服务器端的main.ts文件中，引入TransferState模块和ServerTransferStateModule模块。

import { enableProdMode } from '@angular/core';
import { environment } from './environments/environment';
import { renderModuleFactory } from '@angular/platform-server';
import { AppServerModuleNgFactory } from './path-to-your-server-module';
import { provideModuleMap } from '@nguniversal/module-map-ngfactory-loader';
import { ngExpressEngine } from '@nguniversal/express-engine';
import * as express from 'express';
import { join } from 'path';
import { ServerTransferStateModule } from '@angular/platform-server';

// Enable production mode if required
if (environment.production) {
  enableProdMode();
}

// Create express app
const app = express();

// Set the view engine and specify the folder for views
app.engine(
  'html',
  ngExpressEngine({
    bootstrap: AppServerModuleNgFactory,
    providers: [provideModuleMap(LAZY_MODULE_MAP)],
  })
);
app.set('view engine', 'html');
app.set('views', join(DIST_FOLDER, 'browser'));

// Serve static files from the dist/browser folder
app.get('*.*', express.static(join(DIST_FOLDER, 'browser')));

// Enable TransferState module
app.use(
  '/',
  express.static(join(DIST_FOLDER, 'browser'), {
    index: false,
    maxAge: '1y',
  }),
  ServerTransferStateModule.transferState()
);

// Render the Angular app on each request
app.get('*', (req, res) => {
  res.render('index', { req, res });
});

// Start the server
app.listen(PORT, () => {
  console.log(`Node server listening on http://localhost:${PORT}`);
});

2. 在前端的app.module.ts文件中，导入BrowserTransferStateModule模块。

import { BrowserModule, BrowserTransferStateModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule.withServerTransition({ appId: 'your-app-id' }),
    BrowserTransferStateModule // Import BrowserTransferStateModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

3. 在组件中使用TransferState服务来存储和获取会话状态。

import { Component, OnInit } from '@angular/core';
import { TransferState, makeStateKey } from '@angular/platform-browser';

const SESSION_KEY = makeStateKey('session');

@Component({
  selector: 'app-root',
  template: `
    
Angular Universal Session Example
    Save Session
    Load Session
    
{{ sessionData }}
  `
})
export class AppComponent implements OnInit {
  sessionData: any;

  constructor(private transferState: TransferState) {}

  ngOnInit() {
    // Check if session data exists in transfer state
    if (this.transferState.hasKey(SESSION_KEY)) {
      // Get session data from transfer state
      this.sessionData = this.transferState.get(SESSION_KEY, null);
      // Clear session data from transfer state
      this.transferState.remove(SESSION_KEY);
    }
  }

  saveSession() {
    const data = { username: 'john.doe', token: 'abc123' };
    // Store session data in transfer state
    this.transferState.set(SESSION_KEY, data);
  }

  loadSession() {
    // Get session data from transfer state
    this.sessionData = this.transferState.get(SESSION_KEY, null);
  }
}

通过以上步骤，您的Angular Universal / Node应用的会话状态将能够在客户端和服务器端之间传递，并且每次重新加载页面时都可以访问到相同的会话数据。