Spring in action读书笔记（三） Spring Conditional注解应用-----条件化加载bean

假设有一个名为MagicBean的类，只有设置了magic环境属性的时候，Spring才会实例化这个类。这时可以使用@Conditional注解

package test.aop;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MagicBeanConfig {

    @Bean
    @Conditional(MagicExistsCondition.class)
    public MagicBean magicBean() {
        return new MagicBean();
    }

    static class MagicBean {
        private MagicBean() {
        }
    }
}

@Conditional注解中给定了一个class，它指明了条件，@Conditional接口建通过Condition接口进行条件对比，MagicExistsCondition类实现如下：

package test.aop;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.env.Environment;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class MagicExistsCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        Environment env = context.getEnvironment();
        return env.containsProperty("magic");
    }
}

运行测试类，测试MagicBean能否被正确创建

package test.aop;

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = {MagicBeanConfig.class})
public class TestCondition {

    @Autowired(required = false)
    private MagicBeanConfig.MagicBean magicBean;

    @Test
    public void test() {
        Assert.assertNotEquals(magicBean, null);
    }
}

增加JVM参数  -Dmagic=true  再测试 MagicBean能否被创建