@ConditionalOnProperty:根据属性值来控制类或某个方法是否需要加载。它既可以放在类上也可以放在方法上。
1、SpringBoot实现
1.1 设置配置属性
在applicatio.properties或application.yml配置isload_bean = true;
#配置是否加载类
is_load_bean: true
1.2 编写加载类
编写加载类,使用@Component进行注解,为了便于区分,我们将@ConditionalOnProperty放在方法上。
@Component
@Slf4j
public class UseConditionalOnProperty {
@Value("${is_load_bean}")
private String isLoadBean;
@Bean
@ConditionalOnProperty(value = "is_load_bean",havingValue = "true",matchIfMissing = true)
public void loadBean(){
log.info("是否加载当前类");
}
@Bean
public void compareLoadBean(){
log.info("加载bean属性:" + isLoadBean);
}
}
启动项目时输出打印的日志。如图:
将配置文件的数据信息改成false,则不会打印出结果。
2、ConditionalOnProperty属性与源码
2.1 属性
查看@ConditionalOnProperty源码可以看到该注解定义了几个属性。
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {
String[] value() default {};
String prefix() default "";
String[] name() default {};
String havingValue() default "";
boolean matchIfMissing() default false;
}
2.2 源码
查看OnPropertyCondition类中国的getMatchOutcome()方法:
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
//获取所有注解ConditionalOnProperty下的所有属性match,message
List<AnnotationAttributes> allAnnotationAttributes = annotationAttributesFromMultiValueMap(
metadata.getAllAnnotationAttributes(ConditionalOnProperty.class.getName()));
List<ConditionMessage> noMatch = new ArrayList<>();
List<ConditionMessage> match = new ArrayList<>();
//遍历注解中的属性
for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
//创建判定的结果,ConditionOutcome只有两个属性,
ConditionOutcome outcome = determineOutcome(annotationAttributes, context.getEnvironment());
(outcome.isMatch() ? match : noMatch).add(outcome.getConditionMessage());
}
if (!noMatch.isEmpty()) {
//如果有属性没有匹配,则返回
return ConditionOutcome.noMatch(ConditionMessage.of(noMatch));
}
return ConditionOutcome.match(ConditionMessage.of(match));
}
在上述源码中determineOutcome()是关键方法,我们来看看:
private ConditionOutcome determineOutcome(AnnotationAttributes annotationAttributes, PropertyResolver resolver) {
//初始化
Spec spec = new Spec(annotationAttributes);
List<String> missingProperties = new ArrayList<>();
List<String> nonMatchingProperties = new ArrayList<>();
//收集属性,将结果赋值给missingProperties,nonMatchingProperties
spec.collectProperties(resolver, missingProperties, nonMatchingProperties);
if (!missingProperties.isEmpty()) {
//missingProperties属性不为空,说明设置matchIfMissing的是false,则不加载类
return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
.didNotFind("property", "properties").items(Style.QUOTE, missingProperties));
}
if (!nonMatchingProperties.isEmpty()) {
//nonMatchingProperties属性不为空,则设置的属性值与havingValue值不匹配,则不加载类
return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
.found("different value in property", "different value in properties")
.items(Style.QUOTE, nonMatchingProperties));
}
return ConditionOutcome
.match(ConditionMessage.forCondition(ConditionalOnProperty.class, spec).because("matched"));
}
Spec是OnPropertyCondition的一个静态内部类,初始化@ConditionalOnProperty中的属性。
private static class Spec {
private final String prefix;
private final String havingValue;
private final String[] names;
private final boolean matchIfMissing;
//初始化,给各属性赋值
Spec(AnnotationAttributes annotationAttributes) {
String prefix = annotationAttributes.getString("prefix").trim();
if (StringUtils.hasText(prefix) && !prefix.endsWith(".")) {
prefix = prefix + ".";
}
this.prefix = prefix;
this.havingValue = annotationAttributes.getString("havingValue");
this.names = getNames(annotationAttributes);
this.matchIfMissing = annotationAttributes.getBoolean("matchIfMissing");
}
//处理name与value
private String[] getNames(Map<String, Object> annotationAttributes) {
String[] value = (String[]) annotationAttributes.get("value");
String[] name = (String[]) annotationAttributes.get("name");
//限制了value或name必须指定
Assert.state(value.length > 0 || name.length > 0,
"The name or value attribute of @ConditionalOnProperty must be specified");
//value和name只能有一个存在,不能同时使用
Assert.state(value.length == 0 || name.length == 0,
"The name and value attributes of @ConditionalOnProperty are exclusive");
return (value.length > 0) ? value : name;
}
private void collectProperties(PropertyResolver resolver, List<String> missing, List<String> nonMatching) {
//遍历names,即value或name的值
for (String name : this.names) {
//前缀 + name,获取配置文件中key
String key = this.prefix + name;
//验证配置属性中包含key
if (resolver.containsProperty(key)) {
//如包含,则获取key对应的值,与havingValue的值进行匹配
if (!isMatch(resolver.getProperty(key), this.havingValue)) {
//不匹配则添加到nonMatching
nonMatching.add(name);
}
}
else {
//验证配置属性中没有包含key,判断是否配置了matchIfMissing属性
if (!this.matchIfMissing) {
//该属性不为true则添加到missing中
missing.add(name);
}
}
}
}
private boolean isMatch(String value, String requiredValue) {
//验证requiredValue是否有值
if (StringUtils.hasLength(requiredValue)) {
//有值,则进行比较,不区分大小写
return requiredValue.equalsIgnoreCase(value);
}
//没有值,则验证value是否等于false
//这也是为什么name, value不配置值的情况下, 类依然会被加载的原因
return !"false".equalsIgnoreCase(value);
}
@Override
public String toString() {
StringBuilder result = new StringBuilder();
result.append("(");
result.append(this.prefix);
if (this.names.length == 1) {
result.append(this.names[0]);
}
else {
result.append("[");
result.append(StringUtils.arrayToCommaDelimitedString(this.names));
result.append("]");
}
if (StringUtils.hasLength(this.havingValue)) {
result.append("=").append(this.havingValue);
}
result.append(")");
return result.toString();
}
}
根据业务需求,我们可以实现配置某些属性动态地去加载某些类或方法。
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