其他
那些让你爱不释手的 Spring 代码技巧
1
@Conditional的强大之处
某个功能需要根据项目中有没有某个jar判断是否开启该功能。 某个bean的实例化需要先判断另一个bean有没有实例化，再判断是否实例化自己。 某个功能是否开启，在配置文件中有个参数可以对它进行控制。
@ConditionalOnClass
public class A {}
public class B {}
@ConditionalOnClass(B.class)@Configurationpublic class TestConfiguration {
@Bean public A a() { return new A(); }}@ConditionalOnBean
@Configurationpublic class TestConfiguration {
@Bean public B b() { return new B(); }
@ConditionalOnBean(name="b") @Bean public A a() { return new A(); }}@ConditionalOnProperty
@ConditionalOnProperty(prefix = "demo",name="enable", havingValue = "true",matchIfMissing=true )@Configurationpublic class TestConfiguration {
@Bean public A a() { return new A(); }}demo.enable=falseprefix 表示参数名的前缀，这里是demo name 表示参数名 havingValue 表示指定的值，参数中配置的值需要跟指定的值比较是否相等，相等才满足条件 matchIfMissing 表示是否允许缺省配置。
其他的Conditional注解
自定义Conditional
@Conditional(MyCondition.class)@Retention(RetentionPolicy.RUNTIME)@Target({ElementType.TYPE, ElementType.METHOD})@Documentedpublic @interface MyConditionOnProperty { String name() default "";
String havingValue() default "";}public class MyCondition implements Condition { @Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { System.out.println("实现自定义逻辑"); return false; }}先判断有没有使用Conditional注解，如果没有直接返回false 收集condition到集合中 按order排序该集合 遍历该集合，循环调用condition的matchs方法。
2
如何妙用@Import？
普通类
public class A {}
@Import(A.class)@Configurationpublic class TestConfiguration {}@Autowiredprivate A a;@Configuration注解的配置类
@Import @ImportResource @PropertySource等。
public class A {}
public class B {}
@Import(B.class)@Configurationpublic class AConfiguration {
@Bean public A a() { return new A(); }}
@Import(AConfiguration.class)@Configurationpublic class TestConfiguration {}实现ImportSelector接口的类
public class AImportSelector implements ImportSelector {
private static final String CLASS_NAME = "com.sue.cache.service.test13.A"; public String[] selectImports(AnnotationMetadata importingClassMetadata) { return new String[]{CLASS_NAME}; }}
@Import(AImportSelector.class)@Configurationpublic class TestConfiguration {}实现ImportBeanDefinitionRegistrar接口的类
public class AImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar { @Override public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(A.class); registry.registerBeanDefinition("a", rootBeanDefinition); }}
@Import(AImportBeanDefinitionRegistrar.class)@Configurationpublic class TestConfiguration {}@Import({B.class,AImportBeanDefinitionRegistrar.class})@Configurationpublic class TestConfiguration {}普通类，用于创建没有特殊要求的bean实例。 @Configuration注解的配置类，用于层层嵌套引入的场景。 实现ImportSelector接口的类，用于一次性引入多个类的场景，或者可以根据不同的配置决定引入不同类的场景。 实现ImportBeanDefinitionRegistrar接口的类，主要用于可以手动控制BeanDefinition的创建和注册的场景，它的方法中可以获取BeanDefinitionRegistry注册容器对象。
3
@ConfigurationProperties赋值
thread.pool.corePoolSize=5thread.pool.maxPoolSize=10thread.pool.queueCapacity=200thread.pool.keepAliveSeconds=30public class ThreadPoolConfig {
@Value("${thread.pool.corePoolSize:5}") private int corePoolSize;
@Value("${thread.pool.maxPoolSize:10}") private int maxPoolSize;
@Value("${thread.pool.queueCapacity:200}") private int queueCapacity;
@Value("${thread.pool.keepAliveSeconds:30}") private int keepAliveSeconds;
@Value("${thread.pool.threadNamePrefix:ASYNC_}") private String threadNamePrefix;
@Bean public Executor threadPoolExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(corePoolSize); executor.setMaxPoolSize(maxPoolSize); executor.setQueueCapacity(queueCapacity); executor.setKeepAliveSeconds(keepAliveSeconds); executor.setThreadNamePrefix(threadNamePrefix); executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); executor.initialize(); return executor; }}@Data@Component@ConfigurationProperties("thread.pool")public class ThreadPoolProperties {
private int corePoolSize; private int maxPoolSize; private int queueCapacity; private int keepAliveSeconds; private String threadNamePrefix;}@Configurationpublic class ThreadPoolConfig {
@Autowired private ThreadPoolProperties threadPoolProperties;
@Bean public Executor threadPoolExecutor() { ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(threadPoolProperties.getCorePoolSize()); executor.setMaxPoolSize(threadPoolProperties.getMaxPoolSize()); executor.setQueueCapacity(threadPoolProperties.getQueueCapacity()); executor.setKeepAliveSeconds(threadPoolProperties.getKeepAliveSeconds()); executor.setThreadNamePrefix(threadPoolProperties.getThreadNamePrefix()); executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); executor.initialize(); return executor; }}bindAggregate 绑定集合类 bindBean 绑定对象 bindProperty 绑定参数 前面两种情况最终也会调用到bindProperty方法。
private Map<String, Map<String,String>> urls;@ConditionalOnClass(com.ctrip.framework.apollo.spring.annotation.EnableApolloConfig.class)public class ApolloConfigurationAutoRefresh implements ApplicationContextAware { private ApplicationContext applicationContext; @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { this.applicationContext = applicationContext; } @ApolloConfigChangeListener private void onChange(ConfigChangeEvent changeEvent{ refreshConfig(changeEvent.changedKeys()); } private void refreshConfig(Set<String> changedKeys){ System.out.println("将变更的参数更新到相应的对象中"); }}4
spring事务要如何避坑？
声明式事务
@Servicepublic class UserService {
@Autowired private UserMapper userMapper; @Transactional public void add(UserModel userModel) { userMapper.insertUser(userModel); }}编程式事务
@Servicepublic class UserService { @Autowired private TransactionTemplate transactionTemplate; ... public void save(final User user) { transactionTemplate.execute((status) => { doSameThing... return Boolean.TRUE; }) }}5
跨域问题的解决方案
@RequestMapping("/user")@RestControllerpublic class UserController {
@CrossOrigin(origins = "http://localhost:8016") @RequestMapping("/getUser") public String getUser(@RequestParam("name") String name) { System.out.println("name:" + name); return "success"; }}2. 增加全局配置
@Configurationpublic class WebConfig implements WebMvcConfigurer {
@Override public void addCorsMappings(CorsRegistry registry) { registry.addMapping("/**") .allowedOrigins("*") .allowedMethods("GET", "POST") .allowCredentials(true) .maxAge(3600) .allowedHeaders("*");
}}3. 自定义过滤器
@WebFilter("corsFilter")@Configurationpublic class CorsFilter implements Filter {
@Override public void init(FilterConfig filterConfig) throws ServletException {
}
@Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletResponse httpServletResponse = (HttpServletResponse) response; httpServletResponse.setHeader("Access-Control-Allow-Origin", "*"); httpServletResponse.setHeader("Access-Control-Allow-Methods", "POST, GET"); httpServletResponse.setHeader("Access-Control-Max-Age", "3600"); httpServletResponse.setHeader("Access-Control-Allow-Headers", "x-requested-with"); chain.doFilter(request, response); }
@Override public void destroy() {
}}6
如何自定义starter
在maven仓库找该功能所需jar包 在maven仓库找该jar所依赖的其他jar包 配置新功能所需参数
如果依赖包较多，找起来很麻烦，容易找错，而且要花很多时间。 各依赖包之间可能会存在版本兼容性问题，项目引入这些jar包后，可能没法正常启动。 如果有些参数没有配好，启动服务也会报错，没有默认配置。
它能启动相应的默认配置。 它能够管理所需依赖，摆脱了需要到处找依赖 和 兼容性问题的困扰。 自动发现机制，将spring.factories文件中配置的类，自动注入到spring容器中。 遵循“约定大于配置”的理念。
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion>
<version>1.3.1</version> <groupId>com.sue</groupId> <artifactId>id-generate-spring-boot-starter</artifactId> <name>id-generate-spring-boot-starter</name> <dependencies> <dependency> <groupId>com.sue</groupId> <artifactId>id-generate-spring-boot-autoconfigure</artifactId> <version>1.3.1</version> </dependency> </dependencies></project>pom.xml spring.factories IdGenerateAutoConfiguration IdGenerateService IdProperties pom.xml配置如下：
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>2.0.4.RELEASE</version> </parent> <modelVersion>4.0.0</modelVersion> <version>1.3.1</version> <groupId>com.sue</groupId> <artifactId>id-generate-spring-boot-autoconfigure</artifactId> <name>id-generate-spring-boot-autoconfigure</name>
<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter</artifactId> </dependency>
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-autoconfigure</artifactId> </dependency>
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-configuration-processor</artifactId> <optional>true</optional> </dependency> </dependencies>
<build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <configuration> <source>1.8</source> <target>1.8</target> </configuration> </plugin> </plugins> </build></project>org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.sue.IdGenerateAutoConfiguration@ConditionalOnClass(IdProperties.class)@EnableConfigurationProperties(IdProperties.class)@Configurationpublic class IdGenerateAutoConfiguration {
@Autowired private IdProperties properties;
@Bean public IdGenerateService idGenerateService() { return new IdGenerateService(properties.getWorkId()); }}public class IdGenerateService {
private Long workId;
public IdGenerateService(Long workId) { this.workId = workId; }
public Long generate() { return new Random().nextInt(100) + this.workId; }}@ConfigurationProperties(prefix = IdProperties.PREFIX)public class IdProperties {
public static final String PREFIX = "sue";
private Long workId;
public Long getWorkId() { return workId; }
public void setWorkId(Long workId) { this.workId = workId; }}<dependency> <groupId>com.sue</groupId> <artifactId>id-generate-spring-boot-starter</artifactId> <version>1.3.1</version></dependency>@Autowired private IdGenerateService idGenerateService;7
项目启动时的附加功能
CommandLineRunner ApplicationRunner
@Componentpublic class TestRunner implements ApplicationRunner {
@Autowired private LoadDataService loadDataService;
public void run(ApplicationArguments args) throws Exception { loadDataService.load(); } }CommandLineRunner接口中run方法的参数为String数组 ApplicationRunner中run方法的参数为ApplicationArguments，该参数包含了String数组参数 和 一些可选参数。
以上。
end
“脚本之家”也有了自己的视频号了，
视频号粉丝留言互动
有机会获得红包或者书籍！
推荐阅读：