Spring Boot 4 Microservices & REST APIs · 课时

Bean 作用域

了解单例、原型、请求和会话作用域

第 3 / 4 课13 个步骤

Bean 作用域 是 CoddyKit 上的免费 Spring Boot 4 Microservices & REST APIs 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Spring Boot 4 Microservices & REST APIs 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Spring Boot 4 Microservices & REST APIs 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What a Scope Controls

A bean’s scope determines how many instances the container creates and how long each lives. The default and most common scope is singleton.

Choosing the right scope matters for state, memory, and thread safety.

Singleton Scope

With singleton scope, the container creates exactly one instance per ApplicationContext and returns it for every injection point. This is ideal for stateless services.

@Service // singleton by default
public class PricingService {
    public BigDecimal price(Order o) { /* stateless */ }
}

Singletons Must Be Thread-Safe

Because one singleton serves all concurrent requests, it must not hold mutable per-request state in fields. Keep singletons stateless, or guard shared state carefully.

Good: read-only config, injected collaborators
Bad: a mutable field updated per request

Prototype Scope

Prototype scope creates a new instance every time the bean is requested or injected. Use it for stateful, short-lived helpers that should not be shared.

@Component
@Scope("prototype")
public class ReportBuilder {
    private final List<String> rows = new ArrayList<>();
    public void addRow(String r) { rows.add(r); }
}

The Prototype-in-Singleton Trap

If you inject a prototype into a singleton via the constructor, you get one instance, captured once — not a fresh one per use. The singleton is built only once, so the prototype is resolved only once.

Getting Fresh Prototypes

To obtain a new prototype each time inside a singleton, inject an ObjectProvider (or a lookup) and call it on demand.

@Service
public class ReportService {
    private final ObjectProvider<ReportBuilder> builders;
    public ReportService(ObjectProvider<ReportBuilder> builders) {
        this.builders = builders;
    }
    public Report build() {
        ReportBuilder rb = builders.getObject(); // fresh each call
        return rb.toReport();
    }
}

Web Scopes Overview

In a web application, two extra scopes tie a bean’s lifetime to an HTTP interaction: request and session. They exist only while a request or session is active.

Request Scope

A request-scoped bean is created once per HTTP request and discarded when it completes. Useful for per-request context like a correlation id or accumulated request data.

@Component
@Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class RequestContext {
    private String correlationId;
    // getters/setters
}

Session Scope

A session-scoped bean lives for the duration of a user’s HTTP session, holding per-user state across multiple requests, such as a shopping cart in a server-rendered app.

@Component
@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class UserCart {
    private final List<Item> items = new ArrayList<>();
}

Why Scoped Proxies

Injecting a short-lived scoped bean into a long-lived singleton needs a proxy. The proxy is injected once but delegates each call to the correct current-request or current-session instance.

That is why request/session beans use proxyMode = TARGET_CLASS.

Choosing a Scope

Guidelines:

singleton — default; stateless shared services
prototype — stateful, per-use helpers
request — per-HTTP-request context
session — per-user state across requests

Quick Check

Test your understanding of scopes.

在交互式课程中回答

Recap

Scope controls instance count and lifetime.

singleton (default) — one per context, keep stateless
prototype — new instance per request; beware the singleton trap
request/session — web scopes needing scoped proxies
Use ObjectProvider for fresh prototypes inside singletons

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常见问题解答

「Bean 作用域」课时是免费的吗？

是的 — 「Bean 作用域」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Spring Boot 4 Microservices & REST APIs 课程的其余内容，请升级到 CoddyKit PRO。 Spring Boot 4 Microservices & REST APIs 课程共包含 4 节课。

「Bean 作用域」这节课中我会学到什么？

了解单例、原型、请求和会话作用域你通过在浏览器中直接运行的动手代码来练习 Spring Boot 4 Microservices & REST APIs，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Spring Boot 4 Microservices & REST APIs 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Spring Boot 4 Microservices & REST APIs 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 3 节课，共 4 节。

「Bean 作用域」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Spring Boot 4 Microservices & REST APIs 课中编写并运行代码吗？

能。每节 Spring Boot 4 Microservices & REST APIs 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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