Spring Boot 2.0 遷移指南

此文根據springboot在GitHub上的Spring Boot 2.0 遷移指南文章，做出中文解釋使其通俗易懂，旨在為自己以及各位了解spring、springboot更多的特性與使用方法。
注意，這不是翻譯。

Before You Start

Upgrade to the latest 1.5.x version

Before you start the upgrade, make sure to upgrade to the latest 1.5.x available version. This will make sure that you are building against the most recent dependencies of that line.

官方建議我們先升級到1.5.X版本，運行成功后，再升級2.0，減小升級版本的差異，提升穩定性。

Review dependencies

The move to Spring Boot 2 will upgrade a number of dependencies and might require work on your end. You can review dependency management for 1.5.x with dependency management for 2.0.x to asses how your project is affected.
You may also use dependencies that are not managed by Spring Boot (e.g. Spring Cloud). As your project defines an explicit version for those, you need first to identify the compatible version before upgrading.

大版本升級，很多組件依賴也會隨之升級，需要評估好這些組件間接升級的影響。不過也可以自己引入其他包含自己指定版本組件的依賴，把springboot2.0里的排除掉。

Review custom configuration

Any user-configuration that your project defines might need to be reviewed on upgrade. If this can be replaced by use of standard auto-configuration, do it so before upgrading.

檢查自定義的Config配置會不會受影響，如果可以使用springboot的自動配置能力，就別自定義了，浪費時間還增加springboot版本升級成本。

Review system requirements

Spring Boot 2.0 requires Java 8 or later. Java 6 and 7 are no longer supported. It also requires Spring Framework 5.0.

springboot2.0開始只支持java8及以后版本，spring依賴升級到5.0。

Upgrade to Spring Boot 2

Once you have reviewed the state of your project and its dependencies, upgrade to the latest maintenance release of Spring Boot 2.0. In particular, do not upgrade to Spring Boot 2.0.0.RELEASE as a number of issues have been reported and fixed.
We also recommend to upgrade in phases and not in one jump to the latest GA: first upgrade to 2.0, then 2.1, etc.

做完升級前的檢查后，就可以正式升級了，不過不建議升級到2.0.0.RELEASE，一般來說，2.x表示新特性增加，2.0.x表示各類問題修復，不會引入新特性。所以，可以直接升級到比如2.0.9.RELEASE，2.0x最新的版本。
然后，再逐步升級到2.1.x、2.2.x等等。

Configuration properties migration

With Spring Boot 2.0, many configuration properties were renamed/removed and developers need to update their application.properties/application.yml accordingly. To help you with that, Spring Boot ships a new spring-boot-properties-migrator module. Once added as a dependency to your project, this will not only analyze your application’s environment and print diagnostics at startup, but also temporarily migrate properties at runtime for you. This is a must have during your application migration:
Note:Once you’re done with the migration, please make sure to remove this module from your project’s dependencies.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-properties-migrator</artifactId>
    <scope>runtime</scope>
</dependency>

升級后會有很多配置屬性字段過期或刪除，可以引入這個依賴，啟動項目后，它會輸出可替換的新的配置屬性字段。

Building Your Spring Boot Application

Spring Boot Maven plugin

The plugin configuration attributes that are exposed as properties now all start with a spring-boot prefix for consistency and to avoid clashes with other plugins.
For instance, the following command enables the prod profile using the command line:
mvn spring-boot:run -Dspring-boot.run.profiles=prod

一些命令行發生了變化，比如1.x里：

mvn spring-boot:run -Drun.profiles=prod

2.x里，加上了spring-boot前綴：

mvn spring-boot:run -Dspring-boot.run.profiles=prod

Surefire Defaults

Custom include/exclude patterns have been aligned to latest Surefire’s defaults. If you were relying on ours, update your plugin configuration accordingly. They used to be as follows:
Tip:If you are using JUnit 5, you should upgrade Surefire to 2.22.0.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <includes>
             <include>**/*Tests.java</include>
             <include>**/*Test.java</include>
        </includes>
        <excludes>
            <exclude>**/Abstract*.java</exclude>
        </excludes>
    </configuration>
</plugin>

沒看懂啥意思，平時也不怎么用這個插件。反正就是用JUnit 5，surefire 要升級到 2.22.0。

Spring Boot Gradle Plugin

Spring Boot’s Gradle plugin has been largely rewritten to enable a number of significant improvements. You can read more about the plugin’s capabilities in its reference and api documentation.

springboot的gradle插件進行了重寫改進。

Dependency Management

Spring Boot’s Gradle plugin no longer automatically applies the dependency management plugin. Instead, Spring Boot’s plugin now reacts to the dependency management plugin being applied by importing the correct version of the spring-boot-dependencies BOM. This gives you more control over how and when dependency management is configured.
For most applications applying the dependency management plugin will be sufficient:
Note:The dependency management plugin remains a transitive dependency of spring-boot-gradle-plugin so there’s no need for it to be listed as a classpath dependency in your buildscript configuration.

apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management' // <-- add this to your build.gradle

沒用過gradle，意思應該是 io.spring.dependency-management 插件需要手動添加，而不是自動引入了。

Building Executable Jars and Wars

The bootRepackage task has been replaced with bootJar and bootWar tasks for building executable jars and wars respectively. The jar and war tasks are no longer involved.

老的springboot用gradle打包方式參考https://www.kancloud.cn/george96/java-springboot/613950
2.0里的打包方式參考
Spring Boot Gradle Plugin Reference Guide
即bootRepackage 被替換成了bootJar和bootWar 。

Configuration Updates

The BootRun, BootJar, and BootWar tasks now all use mainClassName as the property to configure the name of the main class. This makes the three Boot-specific tasks consistent with each other, and also aligns them with Gradle’s own application plugin.

mainClassName即啟動類 xxxApplication，現在可以作為屬性進行配置。

Spring Boot Features

Default Proxying Strategy

Spring Boot now uses CGLIB proxying by default, including for the AOP support. If you need interface-based proxy, you’ll need to set the spring.aop.proxy-target-class to false.

springboot2.0默認使用cglib作為動態代理模式，即使類實現了接口，可以通過配置屬性spring.aop.proxy-target-class=false將默認代理切換為jdk。不過spring5.0其實還是默認jdk代理的。

SpringApplication

Web Environment

Spring Boot applications can now operate in more modes so spring.main.web-environment property is now deprecated in favor of spring.main.web-application-type that provides more control.
If you want to make sure an application doesn’t start a web server you’d have to change the property to:
Tip: there is also a setWebApplicationType on SpringApplication if you want to do that programmatically.

spring.main.web-application-type=none

為了適應響應式編程的新潮流，增加了REACTIVE模式。除了配置屬性，也可以編程式實現:

    public static void main(String[] args) {
        SpringApplication springApplication = new SpringApplication(Application.class);
        springApplication.setWebApplicationType(WebApplicationType.NONE);
        springApplication.run(args);
    }

Spring Boot Application Events Changes

We’ve added a new event, ApplicationStartedEvent. ApplicationStartedEvent is sent after the context has been refreshed but before any application and command-line runners have been called. ApplicationReadyEvent is sent after any application and command-line runners have been called. It indicates that the application is ready to service requests.
See the updated reference documentation.

新增了ApplicationStartingEvent，ApplicationXXXEvent中引入了context上下文。這屬于SpringApplicationRunListener 應用監聽器功能相關的改進，具體應用可以看EventPublishingRunListener類設計介紹

Banner

In our effort to limit the number of root namespaces that Spring Boot uses, banner-related properties have been relocated to spring.banner.

這是日志、控制臺打印的文字圖片banner，屬性名稱增加了spring前綴，順便百度看到個動態banner。

Externalized Configuration

Relaxed Binding

The rules related to relaxed binding have been tightened
This new relaxed bindings as several advantages:
1.There is no need to worry about the structure of the key in @ConditionalOnProperty: you must now use the canonical format (acme.my-property and not acme.myProperty), the supported relaxed variants will work transparently. If you were using the prefix attribute you can now simply put the full key using the name or value attributes.
2.RelaxedPropertyResolver is no longer available as the Environment takes care of that automatically: env.getProperty("com.foo.my-bar") will find a com.foo.myBar property.
The org.springframework.boot.bind package is no longer available and is replaced by the new relaxed binding infrastructure. In particular, RelaxedDataBinder and friends have been replaced with a new Binder API. The following samples binds MyProperties from the app.acme prefix.

MyProperties target = Binder.get(environment)
        .bind("app.acme", MyProperties.class)
        .orElse(null);

As relaxed binding is now built-in, you can request any property without having to care about the case as long as it’s using one of the supported formats:

FlagType flagType = Binder.get(environment)
        .bind("acme.app.my-flag", FlagType.class)
        .orElse(FlagType.DEFAULT);

1.@ConditionalOnProperty注解匹配屬性名稱的規則更加嚴格，不再支持駝峰轉換（我試了下，- 和_ 可以互轉，離譜），建議配置屬性長啥樣，匹配名稱就怎么寫好了。
2.RelaxedPropertyResolver類已經刪除了，看了2.0里的實現，一般是用Binder類或Environment類代替實現。
下面也說了，org.springframework.boot.bind整個包都刪了，2.0的寬松綁定規則可以看上面的鏈接，即 new relaxed binding infrastructure。RelaxedDataBinder類也是被Binder類代替實現。

Binding on static methods

While binding on static properties (using a static getter and setter pair) works in Spring Boot 1.x, we never really intended to provide such feature and it is no longer possible as of Spring Boot 2.

這說的應該是給帶有 static 關鍵詞的變量 綁定配置屬性值（例如：springboot讀取yml配置綁定靜態屬性
），官方不推薦這種做法。

@ConfigurationProperties Validation

It is now mandatory that your @ConfigurationProperties object is annotated with @Validated if you want to turn on validation.

屬性配置類里，1.x版本不加@Validated注解，但變量上有@NotNull等校驗規則注解亦能執行校驗，2.0版本開始必須加上@Validated注解才能執行校驗。

Configuration Location

The behavior of the spring.config.location configuration has been fixed; it previously added a location to the list of default ones, now it replaces the default locations. If you were relying on the way it was handled previously, you should now use spring.config.additional-location instead.
讀取jar包外部配置用的，比較奇特，還沒這樣用過，大家可以看下這篇文章spring.config.location與spring.config.additional-location的區別

Developing Web Applications

Embedded containers package structure

In order to support reactive use cases, the embedded containers package structure has been refactored quite extensively. EmbeddedServletContainer has been renamed to WebServer and the org.springframework.boot.context.embedded package has been relocated to org.springframework.boot.web.server. Correspondingly, EmbeddedServletContainerCustomizer has been renamed to WebServerFactoryCustomizer.
For example, if you were customizing the embedded Tomcat container using the TomcatEmbeddedServletContainerFactory callback interface, you should now use TomcatServletWebServerFactory and if you were using an EmbeddedServletContainerCustomizer bean, you should now use a WebServerFactoryCustomizer<TomcatServletWebServerFactory> bean.

為了支持響應式編程，對包做了重構，WebServer新增了NettyWebServer和UndertowWebServer響應式類的web server。WebServerFactoryCustomizer也相應增加了一些實現類。

Servlet-specific server properties

A number of server.* properties that are Servlet-specific have moved to server.servlet:

配置屬性名稱變化.png

Web Starter as a Transitive Dependency

Previously several Spring Boot starters were transitively depending on Spring MVC with spring-boot-starter-web. With the new support of Spring WebFlux, spring-boot-starter-mustache, spring-boot-starter-freemarker and spring-boot-starter-thymeleaf are not depending on it anymore. It is the developer’s responsibility to choose and add spring-boot-starter-web or spring-boot-starter-webflux.

因為spring-boot-starter-webflux的出現，spring-boot-starter-freemarker等依賴不再包含spring-boot-starter-web，由用戶自行引入選擇。

Template Engines

Thymeleaf

Spring Boot 2 uses Thymeleaf 3 which has its own migration guide.

In previous version of Spring Boot, the Thymeleaf starter included the thymeleaf-layout-dialect dependency previously. Since Thymeleaf 3.0 now offers a native way to implement layouts, we removed that mandatory dependency and leave this choice up to you. If your application is relying on the layout-dialect project, please add it explicitly as a dependency.

Thymeleaf 3 有了新的layout實現，把方言依賴去掉了，需要的自己引入。

Mustache Templates Default File Extension

The default file extension for Mustache templates was .html, it is now .mustache to align with the official spec and most IDE plugins. You can override this new default by changing the spring.mustache.suffix configuration key.

改了文件后綴,可以通過配置屬性 spring.mustache.suffix 自定義。

Jackson / JSON Support

In 2.0, we’ve flipped a Jackson configuration default to write JSR-310 dates as ISO-8601 strings. If you wish to return to the previous behavior, you can add spring.jackson.serialization.write-dates-as-timestamps=true to your configuration.
A new spring-boot-starter-json starter gathers the necessary bits to read and write JSON. It provides not only jackson-databind but also useful modules when working with Java8: jackson-datatype-jdk8, jackson-datatype-jsr310 and jackson-module-parameter-names. If you were manually depending on those modules, you can now depend on this new starter instead.

JSR是指Java 規范提案，JSR-310是關于java８的新日期時間API，ISO-8601 是國際標準化組織提供的一個有關時間表示的規范，顯示如1970-01-01T00:00:00Z。springboot2.0里jackson對新日期api序列化做了優化，差異可參考Spring Boot升級到2.x，Jackson對Date時間類型序列化的變化差異，實現原理可參考jackson序列化配置。

Spring MVC Path Matching Default Behavior Change

We’ve decided to change the default for suffix path matching in Spring MVC applications (see #11105). This feature is not enabled by default anymore, following a best practice documented in Spring Framework.
If your application expects requests like "GET /projects/spring-boot.json" to be mapped to @GetMapping("/projects/spring-boot") mappings, this change is affecting you.
For more information about this and how to mitigate that change, check out the reference documentation about path matching and content negotiation in Spring Boot.

GET模式的http請求路徑匹配規則取消了后綴路徑匹配，如 "http://localhost:8080/test.xxx"不能映射到"http://localhost:8080/test"。

Servlet Filters

The default dispatcher types for a Servlet Filter are now DipatcherType.REQUEST; this aligns Spring Boot’s default with the Servlet specification’s default. If you wish to map a filter to other dispatcher types, please register your Filter using a FilterRegistrationBean.
Note：Spring Security and Spring Session filters are configured for ASYNC, ERROR, and REQUEST dispatcher types.

這個對于使用了Servlet Filters的應用會有影響，可以參考SpringBoot1.5.12升級至2.1.9過程中，由DipatcherType引發的ShiroFilter失效問題。

RestTemplateBuilder

The requestFactory(ClientHttpRequestFactory) method has been replaced by a new requestFactory(Supplier<ClientHttpRequestFactory> requestFactorySupplier) method. The use of a Supplier allows every template produced by the builder to use its own request factory, thereby avoiding side-effects that can be caused by sharing a factory. See #11255.

RestTemplateBuilder是一個可用于配置和創建RestTemplate的生成de器。提供便捷方法去注冊converters, error handlers和UriTemplateHandlers。

WebJars Locator

Spring Boot 1.x used and provided dependency management for org.webjars:webjars-locator. webjars-locator is a "poorly named library … that wraps the webjars-locator-core project". Dependencies on org.webjars:webjars-locator should be updated to use org.webjars:webjars-locator-core instead.

WebJars是將web前端資源（js，css等）打成jar包文件，現在都是前后端分離了，應該沒什么人會用了把。

Custom DispatcherServlet registration

If you have customized the registration of the DispatcherServlet, by providing a ServletRegistrationBean named dispatcherServletRegistration, you must also provide a DispatcherServletPath bean so that other components can be aware of the dispatcher servlet’s path. One way to do so is to provide a DispatcherServletRegistrationBean, which implements DispatcherServletPath rather than a ServletRegistrationBean when customizing the registration.

ServletRegistrationBean是將Servlet注冊到spring用的，沒看懂這段話什么意思。

Security

Tip：This section provides a summary of the changes to security in Spring Boot 2. If you want to know more, refer to the Security migration use cases wiki page.

Spring Boot 2 greatly simplifies the default security configuration and makes adding custom security easy. Rather than having several security-related auto-configurations, Spring Boot now has a single behavior that backs off as soon as you add your own WebSecurityConfigurerAdapter.
You are affected if you were using any of the following properties:

security.basic.authorize-mode
security.basic.enabled
security.basic.path
security.basic.realm
security.enable-csrf
security.headers.cache
security.headers.content-security-policy
security.headers.content-security-policy-mode
security.headers.content-type
security.headers.frame
security.headers.hsts
security.headers.xss
security.ignored
security.require-ssl
security.sessions

Default Security

The security auto-configuration no longer exposes options and uses Spring Security defaults as much as possible. One noticeable side effect of that is the use of Spring Security’s content negotiation for authorization (form login).
Spring Boot 2.0 doesn’t deviate too much from Spring Security’s defaults, as a result of which some of the endpoints that bypassed Spring Security in Spring Boot 1.5 are now secure by default. These include the error endpoint and paths to static resources such as /css/**, /js/**, /images/**, /webjars/**, /**/favicon.ico. If you want to open these up, you need to explicitly configure that.

springboot2.0默認配置了安全屬性，可以通過繼承WebSecurityConfigurerAdapter自定義安全配置。

Default User

Spring Boot configures a single user with a generated password, by default. The user can be configured using properties under spring.security.user.*. To customize the user further or add other users, you will have to expose a UserDetailsService bean instead. This sample demonstrates how to do it.

在SecurityProperties類中定義了默認用戶的屬性，可以通過屬性配置或者注冊InMemoryUserDetailsManager Bean，替換默認實現生成自定義的用戶。

To disable default user creation, provide a bean of type AuthenticationManager, AuthenticationProvider or UserDetailsService.
Note：Autowiring an AuthenticationManagerBuilder into a method in a configuration class does not disable creation of the default user.

一般來說，正常應用都是從數據庫或其他地方讀取用戶信息，而不是一個默認用戶。參考AuthenticationManager、AuthenticationProvider、UserDetailsService的關系與區別了解AuthenticationManager, AuthenticationProvider和UserDetailsService。

AuthenticationManager Bean

If you want to expose Spring Security’s AuthenticationManager as a bean, override the authenticationManagerBean method on your WebSecurityConfigurerAdapter and annotate it with @Bean.

如果想要將AuthenticationManager聲明為Bean，需要覆寫WebSecurityConfigurerAdapter類的authenticationManagerBean方法，再加上@Bean注解。

OAuth2

Functionality from the Spring Security OAuth project is being migrated to core Spring Security. Dependency management is no longer provided for that dependency and Spring Boot 2 provides OAuth 2.0 client support via Spring Security 5.
If you depend on Spring Security OAuth features that have not yet been migrated, you will need to add a dependency on an additional jar, check the documentation for more details. We’re also continuing to support Spring Boot 1.5 so older applications can continue to use that until an upgrade path is provided.

依賴合并，沒什么好說的。

Actuator Security

There is no longer a separate security auto-configuration for the Actuator (management.security.* property are gone). The sensitive flag of each endpoint is also gone to make things more explicit in the security configuration. If you were relying to this behavior, you need to create or adapt your security configuration to secure endpoints with the role of your choice.
For instance, assuming the following config

endpoints.flyway.sensitive=false
endpoints.info.sensitive=true
management.security.roles=MY_ADMIN

這是配置監控端點安全的，這些配置屬性已經不能用了。

http
    .authorizeRequests()
    .requestMatchers(EndpointRequest.to("health", "flyway")).permitAll()
        .requestMatchers(EndpointRequest.toAnyEndpoint()).hasRole("MY_ADMIN")
        ...

可以通過繼承WebSecurityConfigurerAdapter類實現，參考SpringBoot之Actuator。

Note that in 2.x, health and info are enabled by default (with health details not shown by default). To be consistent with those new defaults, health has been added to the first matcher.

Working with SQL Databases

Spring Data Kay renamed a number of its CRUD repository methods. Application code calling the renamed methods will have to be updated. To ease the migration, you may want to consider using a custom CrudRepository sub-interface that declares deprecated default methods that use the old names and delegate to the equivalent newly named method. Marking the default methods has deprecated will help to ensure that the migration is not forgotten.

我也不知道什么方法重命名了，找不到，無語。

Configuring a DataSource

The default connection pool has switched from Tomcat to HikariCP. If you used spring.datasource.type to force the use of Hikari in a Tomcat-based application, you can now remove that override.

默認數據源已切換到HikariCP，不需要再用配置屬性強制指定。

In particular, if you had such setup:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>tomcat-jdbc</artifactId>
        </exclusion>
    </exclusions>
</dependency>

<dependency>
    <groupId>com.zaxxer</groupId>
    <artifactId>HikariCP</artifactId>
</dependency>

you can now replace it with:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

跟上面一樣的意思，已經默認加了。

WARN Message for Implicit 'open-in-view'

From now on, applications that don’t explicitly enable spring.jpa.open-in-view will get a WARN message during startup. While this behavior is a friendly default, this can lead to issues if you’re not fully aware of what’s that doing for you. This message makes sure that you understand that database queries may be performed during view rendering. If you’re fine with that, you can configure explicitly this property to silence the warning message.

沒有顯式配置則默認為true，應用啟動時打印warn日志。這個屬性的作用是添加一個攔截器OpenEntityManagerInViewInterceptor，它會改變Session的創建時機和事務的范圍，可以參考JPA的使用總結。

JPA

In Spring Boot 1.x, some users were extending from HibernateJpaAutoConfiguration to apply advanced customizations to the auto-configured EntityManagerFactory. To prevent such faulty use case from happening, it is no longer possible to extend from it in Spring Boot 2.

應該是指org.hibernate.jpa.HibernateEntityManagerFactory過期了的事。

To support those use cases, you can now define a HibernatePropertiesCustomizer bean that gives you full control over Hibernate properties, including the ability to register Hibernate interceptor declared as beans in the context.

應該是要實現HibernatePropertiesCustomizer接口，聲明注解@Configuration，沒有實踐過。

Id generator

The spring.jpa.hibernate.use-new-id-generator-mappings property is now true by default to align with the default behaviour of Hibernate. If you need to temporarily restore this now deprecated behaviour, set the property to false.

use-new-id-generator-mappings以前默認為false，現在默認為true，是用來判斷選擇主鍵生成策略的，可以參考# SequenceStyleGenerator MySQL 使用注意事項

Flyway

Flyway configuration keys were moved to the spring namespace (i.e. spring.flyway)
Upgrading to Spring Boot 2 will upgrade Flyway from 3.x to 5.x. To make sure that the schema upgrade goes smoothly, please follow the following instructions:

• First upgrade your 1.5.x Spring Boot application to Flyway 4 (4.2.0 at the time of writing), see the instructions for Maven and Gradle.
• Once your schema has been upgraded to Flyway 4, upgrade to Spring Boot 2 and run the migration again to port your application to Flyway 5.

If you experience a checksum error on upgrading the schema (i.e. FlywayException: Validate failed. Migration Checksum mismatch), invoking repair could help as show in the following example
Tip：Alternatively, this blog post by @wimdeblauwe provides a different approach that saves the two step upgrade.

@Bean
public FlywayMigrationStrategy repairStrategy() {
    return flyway -> {
        flyway.repair();
        flyway.migrate();
    };
}

Flyway 數據庫版本控制的組件，第一次見識。參考介紹Flyway 數據庫版本控制。
增加了配置屬性的前綴spring。

Liquibase

Liquibase configuration keys were moved to the spring namespace (i.e. spring.liquibase)

增加了配置屬性的前綴spring。

Liquibase

Liquibase configuration keys were moved to the spring namespace (i.e. spring.liquibase)

Liquibase 也是數據庫版本控制的組件，同樣沒用過。參考介紹數據庫版本控制-liquibase

Database Initialization

Basic DataSource initialization is now only enabled for embedded data sources and will switch off as soon as you’re using a production database. The new spring.datasource.initialization-mode (replacing spring.datasource.initialize) offers more control.

這個屬性是用來判斷是否執行初始化sql的。拓展知識springboot 啟動時初始化數據庫的步驟

Updated Default 'create-drop' Handling

The spring.jpa.hibernate.ddl-auto property defaults to create-drop with an embedded database only if no schema manager, such as Liquibase or Flyway, is in use. As soon as a schema manager is detected, the default changes to none.

spring.jpa.hibernate.ddl-auto 實際對應屬性 hibernate.hbm2ddl.auto，用來控制表結構的更改，甚至創建和刪除。當連接嵌入式數據庫時，默認 create-drop，其他默認none。
拓展知識Spring Boot初始化數據庫和導入數據以及spring.jpa.generate-dll與spring.jpa.hibernate.ddl-auto之間的困惑。

Working with NoSQL Technologies

Redis

Lettuce is now used instead of Jedis as the Redis driver when you use spring-boot-starter-data-redis. If you are using higher level Spring Data constructs you should find that the change is transparent.
We still support Jedis. Switch dependencies if you prefer Jedis by excluding io.lettuce:lettuce-core and adding redis.clients:jedis instead.

redis客戶端依賴Jedis默認替換為Lettuce。

Connection pooling is optional and, if you are using it, you now need to add commons-pool2 yourself as Lettuce, contrary to Jedis, does not bring it transitively.

Jedis是直連redis的，所以一個請求對應一次連接開關，比較消耗性能，所以配置連接池很有必要。Lettuce是基于netty的連接實例，不需要連接池也能共享連接，如果連接池使用不當，反而還會降低性能。拓展知識lettuce連接池真有必要嗎？。

Elasticsearch

Elasticsearch has been upgraded to 5.4+. In line with Elastic’s announcement that embedded Elasticsearch is no longer supported, auto-configuration of a NodeClient has been removed. A TransportClient can be auto-configured by using spring.data.elasticsearch.cluster-nodes to provide the addresses of one or more nodes to connect to.

官方不再支持嵌入式Elasticsearch ，因為它繞過了安全管理、插件加載等機制。節點配置方式發生了改變。

Caching

Dedicated Hazelcast Auto-configuration for Caching

It is no longer possible to auto-configure both a general HazelcastInstance and a dedicated HazelcastInstance for caching. As a result, the spring.cache.hazelcast.config property is no longer available.

Hazelcast是一個基于內存的計算存儲平臺，擅長流數據處理，沒用過，不多做解釋。

GuavaCacheManager

Support for Guava has been dropped in Spring Framework 5. If you were are using GuavaCacheManager, Caffeine (com.github.ben-manes.caffeine:caffeine) and CaffeineCacheManager should be used instead.

在spring5中，有以下幾種本地緩存實現：
CaffeineCache：基于Caffeine組件實現。
ConcurrentMapCache：基于java的ConcurrentMap實現。
EhCacheCache：基于EhCache組件實現。
JCacheCache：基于javax.cache實現。
NoOpCache：禁用緩存操作實現。這個實現的目的在于通過編寫配置的方式便捷切換緩存的使用與禁用，如@Profeile。
TransactionAwareCacheDecorator：緩存裝飾器，具體實現由注入的Cache決定。需要配合spring事務使用，在事務提交后執行cache操作，相當于Read Committed級別。拓展知識TransactionAwareCacheDecorator升級版。

RedisCacheManager

The Redis CacheManager implementation has been reworked significantly, make sure to review the reference documentation.

RedisCacheManager被重寫了，現在可以將其配置為Bean，方便用戶自定義。

Batch

The CommandLineRunner that executes batch jobs on startup has an order of 0.

這是指實現類JobLauncherCommandLineRunner，現在實現了Ordered接口，默認orderrder順序為0。

Testing

Mockito 1.x

Mockito 1.x is no longer supported for @MockBean and @SpyBean. If you don’t use spring-boot-starter-test to manage your dependencies you should upgrade to Mockito 2.x.
Note：See also What’s new in Mockito 2

沒用過Mockito，使用參考SpringBoot 單元測試詳解（Mockito、MockBean）。

EnvironmentTestUtils

EnvironmentTestUtils is deprecated in favour of TestPropertyValues that offers a similar, yet more powerful API as demonstrated in the following example:

TestPropertyValues.of("acme.first=1", "acme.second=2")
        .and("acme.third=3")
        .applyTo(this.environment);

EnvironmentTestUtils 應替換為 TestPropertyValues。

Creating Your Own Auto-configuration

ConditionalOnBean semantic change

ConditionalOnBean is now using a logical AND rather than an OR for candidate beans. If you need to keep a condition where any of the target beans are present, consider using a AnyNestedCondition as shown in the following example:

class ThisOrThatCondition extends AnyNestedCondition {

    ThisOrThatCondition() {
        super(ConfigurationPhase.REGISTER_BEAN);
    }

    @ConditionalOnBean(This.class)
    static class ThisCondition {

    }

    @ConditionalOnBean(That.class)
    static class ThatCondition {

    }

}

ConditionalOnBean 的條件語義發生變化，當設置了多個類時，如@ConditionalOnBean(xxx.class,xxx.class)，OR條件改為AND條件。

Spring Boot Actuator

Spring Boot 2 brings important changes to the actuator, both internal and user-facing, please check the updated section in the reference guide and the new Actuator API documentation.

監控平臺升級沒啥好說的，參考介紹springboot 的指標監控。

Build

The code of the Actuator has been split in two modules: the existing spring-boot-actuator and a new spring-boot-actuator-autoconfigure. If you were importing the actuator using its original module (spring-boot-actuator), please consider using the spring-boot-starter-actuator starter instead.

actuator依賴升級，建議更換新的依賴。

Configuration Keys Structure

Endpoints infrastructure key have been harmonized:

配置屬性

配置屬性新老交替，替換下就好了。

Base Path

All endpoints have moved to /actuator by default.
We fixed the meaning of management.server.servlet.context-path: it is now the endpoint management equivalent of server.servlet.context-path (only active when management.server.port is set). Additionally, you can also set the base path for the management endpoints with a new, separate property: management.endpoints.web.base-path.
For example, if you’ve set management.server.servlet.context-path=/management and management.endpoints.web.base-path=/application, you’ll be able to reach the health endpoint at the following path: /management/application/health.

actuator 的端點基礎路徑默認為 /actuator，如http://localhost:8080/actuator/health；當 management.endpoints.web.base-path=/manage，那么http://localhost:8080/manage/health；當management.server.servlet.context-path=/management 單獨使用，無法生效，要加上 management.server.port=8082，如http://localhost:8082/management/actuator/health；當 management.endpoints.web.base-path=/application，那么 http://localhost:8082/application/health；三條屬性同時配置，那么 http://localhost:8082/management/application/health。

If you want to restore the behavior of 1.x (i.e. having /health instead of /actuator/health), set the following property:

management.endpoints.web.base-path=/

通過此配置，可以還原到actuator 1.x的路徑格式。

Audit Event API Change

AuditEventRepository now has a single method with all optional arguments.

AuditEventRepository 用來添加AuditEvent的。AuthenticationAuditListener定義了三個事件用于監聽，AUTHENTICATION_SUCCESS、AUTHENTICATION_FAILURE、AUTHENTICATION_SWITCH，并要求配合spring security使用。用戶可以在端點 /auditevents 下查看審計事件。參考Springboot Actuator之十：actuator中的audit包和Spring Boot Actuator。

Endpoints

To make an actuator endpoint available via HTTP, it needs to be both enabled and exposed. By default:

• Only the /health and /info endpoints are exposed, regardless of Spring Security being present and configured in your application.
• All endpoints but /shutdown are enabled.

端點需要啟用并公開才能訪問，默認只公開 /health 和 /info，除 /shutdown 外其他端點都是啟用的。

You can expose all endpoints as follows:

management.endpoints.web.exposure.include=*

公開所有端點。

You can explicitly enable the /shutdown endpoint with:

management.endpoint.shutdown.enabled=true

啟用 /shutdown 端點。

To expose all (enabled) web endpoints but the env endpoint:

management.endpoints.web.exposure.include=*
management.endpoints.web.exposure.exclude=env

公開除env端點之外的所有（已啟用）web端點。

端點變更

Endpoint properties have changed as follows:

• endpoints.<id>.enabled has moved to management.endpoint.<id>.enabled
• endpoints.<id>.id has no replacement (the id of an endpoint is no longer configurable)
• endpoints.<id>.sensitive has no replacement (See Actuator Security)
• endpoints.<id>.path has moved to management.endpoints.web.path-mapping.<id>

端點配置屬性變化。

Endpoint Format

Overhaul of the "/actuator/mappings" Actuator Endpoint

The JSON format has changed to now properly include information about context hierarchies, multiple DispatcherServlets, deployed Servlets and Servlet filters. See #9979 for more details.
The relevant section of the Actuator API documentation provides a sample document.

/mappings 端點可以查看 @RequestMapping 下的請求路徑映射的json列表，2.x的json格式變了，看不出有啥影響。

Overhaul of the "/actuator/httptrace" Actuator Endpoint

The structure of the response has been refined to reflect the endpoint’s focus on tracing HTTP request-response exchanges. More details about the endpoint and its response structure can be found in the relevant section of the Actuator API documentation.

/httptrace 端點可以查看http請求信息，2.x的json格式變了。

Migrate Custom Endpoints

If you have custom actuator endpoints, please check out the dedicated blog post. The team also wrote a wiki page that describes how to migrate your existing Actuator endpoints to the new infrastructure.

1.x和2.x的端點實現方式變了很多，參考actuator 2.x 自定義實現端點。

Metrics

Spring Boot’s own metrics have been replaced with support, including auto-configuration, for Micrometer and dimensional metrics.

Setting up Micrometer

If your Spring Boot 2.0 application already depends on Actuator, Micrometer is already here and auto-configured. If you wish to export metrics to an external registry like Prometheus, Atlas or Datadog, Micrometer provides dependencies for many registries; you can then configure your application with spring.metrics.* properties to export to a particular registry.
For more on this, check out the Micrometer documentation about Spring Boot 2.0.

Micrometer 是一個基于 JVM 的應用程序指標收集工具包，提供了抽象接口和脫離底層的第三方監控依賴，類似于 SLF4J 在 Java 日志中的作用。參考介紹Spring Boot集成Micrometer。

Migrating Custom Counters/Gauges

Instead of injecting CounterService or GaugeService instances in your application code, you can create various metrics by:

• Injecting a MeterRegistry and calling methods on it.
• Directly calling static methods like Counter featureCounter = Metrics.counter("feature");.
  Micrometer brings many interesting features - check out the core concepts behind Micrometer and the specifics about Spring Boot integration.

可以參考上面的鏈接文章。

Spring Boot 1.5 Support

You can plug existing Spring Boot 1.5 applications in the same metrics infrastructure by using the Micrometer Legacy support.

網站打不開，后面再看。

Developer Tools

Hot swapping

As the Spring Loaded project has been moved to the attic, its support in Spring Boot has been removed. We advise to use Devtools instead.

Spring Loaded項目被擱置，推薦spring-boot-devtools。
Spring Loaded自2017年9月后，版本就沒有升級過了，看來是被放棄了。

Devtools Remote Debug Tunnel

The support for tunnelling remote debugging over HTTP has been removed from Devtools.

devtools 通過 HTTP 進行遠程調試，這個功能被刪除了。

Removed Features

The following features are no longer available:

• CRaSH support
• Auto-configuration and dependency management for Spring Mobile.
• Auto-configuration and dependency management for Spring Social. Please check the Spring Social project for more details.
• Dependency management for commons-digester.
• Test support in the CLI (i.e. spring test)

CRaSH support：遠程執行命令。
Auto-configuration and dependency management for Spring Mobile：spring-mobile是基于模板引擎開發的框架，在前后端分離趨勢下，項目廢棄是理所當然的。
Auto-configuration and dependency management for Spring Social. Please check the Spring Social project for more details：為什么放棄spring social項目及替代方案。
Dependency management for commons-digester：ommons-digester工具包的作用是將xml轉換為java對象。
Test support in the CLI (i.e. spring test)：我的理解是spring-test不支持spring-boot-CLI。Spring Boot CLI是Spring Boot的命令行界面，使用groovy語言編寫，它可以用來快速啟動Spring。

Dependency Versions

The minimum supported version of the following libraries has changed:

• Elasticsearch 5.6
• Gradle 4
• Hibernate 5.2
• Jetty 9.4
• Spring Framework 5
• Spring Security 5
• Spring Integration 5 (see also their migration guide)
• Tomcat 8.5

最小依賴版本支持。

總結

這篇文章的編寫耗時超過了我的預期，涉及知識點廣泛，想要盡可能地理解每一點，需要花不少時間，即使如此，依然有很多理解偏頗（希望有人能指正）。不過寫完后，發現自己也的確有所收獲。不管在面試還是日常工作中，我們都需要知道自己所用何物，才能物盡其用，所以當工具發生變化時，及時了解變化之所在，是很有必要的。

遷移指南中文翻譯

https://blog.51cto.com/u_14299052/2935938