【Java Web】条件Bean@ConditionalOnResource

【Java Web】条件Bean@ConditionalOnResource

注解仅仅会加载classpath中指定的资源，资源存在则创建对象，资源不存在则不创建对象。外部资源文件无法加载，想要使用外部资源文件参考：

resources目录下资源application.properties，存在则创建对象

import org.springframework.boot.autoconfigure.condition.ConditionalOnResource;
import org.springframework.context.annotation.Configuration;

@ConditionalOnResource(resources = {"application.properties"})
@Configuration
public class TestConditionalOnResource {
}

@Autowired(required = false)
TestConditionalOnResource bean;

实现类

/*
 * Copyright 2012-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.autoconfigure.condition;

import java.util.ArrayList;
import java.util.List;

import org.springframework.boot.autoconfigure.condition.ConditionMessage.Style;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.util.Assert;
import org.springframework.util.MultiValueMap;

/**
 * {@link Condition} that checks for specific resources.
 *
 * @author Dave Syer
 * @see ConditionalOnResource
 */
@Order(Ordered.HIGHEST_PRECEDENCE + 20)
class OnResourceCondition extends SpringBootCondition {

    @Override
    public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        MultiValueMap<String, Object> attributes = metadata
                .getAllAnnotationAttributes(ConditionalOnResource.class.getName(), true);
        ResourceLoader loader = context.getResourceLoader();
        List<String> locations = new ArrayList<>();
        collectValues(locations, attributes.get("resources"));
        Assert.isTrue(!locations.isEmpty(),
                "@ConditionalOnResource annotations must specify at least one resource location");
        List<String> missing = new ArrayList<>();
        for (String location : locations) {
            String resource = context.getEnvironment().resolvePlaceholders(location);
            if (!loader.getResource(resource).exists()) {
                missing.add(location);
            }
        }
        if (!missing.isEmpty()) {
            return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnResource.class)
                    .didNotFind("resource", "resources").items(Style.QUOTE, missing));
        }
        return ConditionOutcome.match(ConditionMessage.forCondition(ConditionalOnResource.class)
                .found("location", "locations").items(locations));
    }

    private void collectValues(List<String> names, List<Object> values) {
        for (Object value : values) {
            for (Object item : (Object[]) value) {
                names.add((String) item);
            }
        }
    }

}