Spring Boot中的那些条件判断的实现方法
Spring Boot中的那些Conditional
spring boot中为我们提供了丰富的Conditional来让我们得以非常方便的在项目中向容器中添加Bean。本文主要是对各个注解进行解释并辅以代码说明其用途。
所有ConditionalOnXXX的注解都可以放置在class或是method上,如果方式在class上,则会决定该class中所有的@Bean注解方法是否执行。
@Conditional
下面其他的Conditional注解均是语法糖,可以通过下面的方法自定义ConditionalOnXXX
Conditional注解定义如下,接收实现Condition接口的class数组。
public @interface Conditional { Class<? extends Condition>[] value(); }
而Condition接口只有一个matchs方法,返回是否匹配的结果。
public interface Condition { boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }
通过操作系统进行条件判断,从而进行Bean配置。当Window时,实例化Bill的Person对象,当Linux时,实例化Linus的Person对象。
//LinuxCondition,为方便起见,去掉判断代码,直接返回true了 public class LinuxCondition implements Condition { @Override public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) { return true; } }
//WindowsCondition,为方便起见,去掉判断代码,直接返回false了 public class WindowsCondition implements Condition { @Override public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata metadata) { return false; } } @Data
@ToString @AllArgsConstructor @NoArgsConstructor public class Person { private String name; private Integer age; }
//配置类 @Configuration public class BeanConfig { @Bean(name = "bill") @Conditional({WindowsCondition.class}) public Person person1(){ return new Person("Bill Gates",62); } @Bean("linus") @Conditional({LinuxCondition.class}) public Person person2(){ return new Person("Linus",48); } }
public class AppTest { AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(BeanConfig.class); @Test public void test(){ String osName = applicationContext.getEnvironment().getProperty("os.name"); System.out.println("当前系统为:" + osName); Map<String, Person> map = applicationContext.getBeansOfType(Person.class); System.out.println(map); } }
输出的结果:
当前系统为:Mac OS X
{linus=Person(name=Linus, age=48)}
@ConditionalOnBean & @ConditionalOnMissingBean
这两个注解会对Bean容器中的Bean对象进行判断,使用的例子是配置的时候,如果发现如果没有Computer实例,则实例化一个备用电脑。
@Data @AllArgsConstructor @ToString public class Computer { private String name; }
@Configuration public class BeanConfig { @Bean(name = "notebookPC") public Computer computer1(){ return new Computer("笔记本电脑"); } @ConditionalOnMissingBean(Computer.class) @Bean("reservePC") public Computer computer2(){ return new Computer("备用电脑"); } }
public class TestApp { AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(BeanConfig.class); @Test public void test1(){ Map<String,Computer> map = applicationContext.getBeansOfType(Computer.class); System.out.println(map); } }
修改BeanConfig,如果注释掉第一个@Bean,会实例化备用电脑,否则就不会实例化备用电脑
@ConditionalOnClass & @ConditionalOnMissingClass
这个注解会判断类路径上是否有指定的类,一开始看到的时候比较困惑,类路径上如果没有指定的class,那编译也通过不了啊...这个主要用于集成相同功能的第三方组件时用,只要类路径上有该组件的类,就进行自动配置,比如spring boot web在自动配置视图组件时,是用Velocity,还是Thymeleaf,或是freemaker时,使用的就是这种方式。
例子是两套盔甲A(光明套装)和B(暗黑套装),如果A不在则配置B。
public interface Fighter { void fight(); } public class FighterA implements Fighter { @Override public void fight() { System.out.println("使用光明套装"); } } public class FighterB implements Fighter { @Override public void fight() { System.out.println("使用暗黑套装"); } }
Van是武士,使用套装进行战斗
@Data @AllArgsConstructor @NoArgsConstructor public class Van { private Fighter fighter; public void fight(){ fighter.fight(); } }
VanConfigA/B实例化武士
@Configuration @ConditionalOnClass({FighterA.class}) public class VanConfigA { @Primary @Bean public Van vanA(){ return new Van(new FighterA()); } } @Configuration @ConditionalOnClass({FighterB.class}) public class VanConfigB { @Bean public Van vanB(){ return new Van(new FighterB()); } }
测试类,默认情况,如果套装AB都在类路径上,两套都会加载,A会设置为PRIMARY,如果在target class中将FightA.class删除,则只会加载套装B。
@SpringBootApplication public class TestApp implements CommandLineRunner { @Autowired private Van van; public static void main(String[] args) { SpringApplication.run(TestApp.class, args); } @Override public void run(String... args) throws Exception { //do something van.fight(); } }
另外,尝试将两个VanConfigA/B合并,将注解ConditionalOnClass放到方法上,如果删除一个套装就会运行出错。
@ConditionalOnExpress
依据表达式进行条件判断,这个作用和@ConditionalOnProperty大部分情况可以通用,表达式更灵活一点,因为可以使用SpEL。例子中会判断properties中test.enabled的值进行判断。BeanConfig分别对布尔,字符串和数字三种类型进行判断。数字尝试了很多其他的方式均不行,比如直接使用==,貌似配置的属性都会当成字符串来处理。
@Data public class TestBean { private String name; }
@Configuration @ConditionalOnExpression("#{${test.enabled:true} }") //@ConditionalOnExpression("'zz'.equalsIgnoreCase('${test.name2}')") //@ConditionalOnExpression("new Integer('${test.account}')==1") public class BeanConfig { @Bean public TestBean testBean(){ return new TestBean("我是美猴王"); } }
@SpringBootApplication public class TestAppCommand implements CommandLineRunner { @Autowired private TestBean testBean; public static void main(String[] args) { SpringApplication.run(TestAppCommand.class, args); } @Override public void run(String... args) throws Exception { System.out.println(testBean.getName()); } }
@ConditionalOnProperty
适合对单个Property进行条件判断,而上面的@ConditionalOnExpress适合面对较为复杂的情况,比如多个property的关联比较。这个例子也给了三种基本类型的条件判断,不过貌似均当成字符串就可以...
@Data @AllArgsConstructor @NoArgsConstructor public class TestBean { private String name; }
@Configuration @ConditionalOnProperty(prefix = "test", name="enabled", havingValue = "true",matchIfMissing = false) //@ConditionalOnProperty(prefix = "test", name="account", havingValue = "1",matchIfMissing = false) //@ConditionalOnProperty(prefix = "test", name="name1", havingValue = "zz",matchIfMissing = false) public class BeanConfig { @Bean public TestBean testBean(){ return new TestBean("我是美猴王"); } }
@SpringBootApplication public class TestAppCommand implements CommandLineRunner { @Autowired private TestBean testBean; public static void main(String[] args) { SpringApplication.run(TestAppCommand.class, args); } @Override public void run(String... args) throws Exception { System.out.println(testBean.getName()); } }
@ConditionalOnJava
可以通过java的版本进行判断。
@Data public class TestBean { }
@Configuration @ConditionalOnJava(JavaVersion.EIGHT) public class BeanConfig { @Bean public TestBean testBean(){ return new TestBean(); } }
public class TestApp { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BeanConfig.class); @Test public void test(){ Map<String,TestBean> map = context.getBeansOfType(TestBean.class); System.out.println(map); } }
@ConditionalOnResource
通过指定的资源文件是否存在进行条件判断,比如判断ehcache.properties来决定是否自动装配ehcache组件。
@Data public class TestBean { }
@Configuration @ConditionalOnResource(resources = "classpath:application.yml") public class BeanConfig { @Bean public TestBean testBean(){ return new TestBean(); } }
public class TestApp { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BeanConfig.class); @Test public void test(){ Map<String,TestBean> map = context.getBeansOfType(TestBean.class); System.out.println(map); } }
@ConditionalOnSingleCandidate
这个还没有想到应用场景,条件通过的条件是:1 对应的bean容器中只有一个 2.对应的bean有多个,但是已经制定了PRIMARY。例子中,BeanB装配的时候需要看BeanA的装配情况,所以BeanBConfig要排在BeanAConfig之后.可以修改BeanAConfig,将@Primary注解去掉,或者把三个@Bean注解去掉,BeanB就不会实例化了。
@Data @AllArgsConstructor @NoArgsConstructor public class BeanA { private String name; }
@Configuration public class BeanAConfig { @Bean @Primary public BeanA bean1(){ return new BeanA("bean1"); } @Bean(autowireCandidate = false) public BeanA bean2(){ return new BeanA("bean2"); } //@Bean(autowireCandidate = false) public BeanA bean3(){ return new BeanA("bean3"); } }
@Data public class BeanB { }
@Configuration @AutoConfigureAfter(BeanAConfig.class) @ConditionalOnSingleCandidate(BeanA.class) public class BeanBConfig { @Bean public BeanB targetBean(){ return new BeanB(); } }
public class TestApp { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BeanAConfig.class, BeanBConfig.class); @Test public void test(){ Map<String,BeanA> map = context.getBeansOfType(BeanA.class); System.out.println(map); Map<String,BeanB> map2 = context.getBeansOfType(BeanB.class); System.out.println(map2); } }
@ConditionalOnNotWebApplication & @ConditionalOnWebApplication
判断当前环境是否是Web应用。
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