Spring Boot中的那些条件判断

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：

@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。