早期，如果需要通过spring读取properties文件中的配置信息，都需要在XML文件中配置文件读取方式。
基于XML的读取方式：
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<value>classpath:properties/thread-pool.properties</value>
</list>
</property>
</bean>
当然，这种方式可以统一管理properties配置文件，也能实现代码的松耦合。但为了方便开发，提高开发效率，spring官方后来提供了基于注解的配置读取方式。两种方式各有优势，可以基于对项目的考虑选择最合适的方式。接下来就介绍如何通过注解注入properties的配置信息。
首先，准备配置文件：
core.pool.size=2
max.pool.size=3
keep.alive.time=1
task.queue.size=3
await.termination.time=5
定义配置类：
package org.cellphone.config; import com.google.gson.Gson;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.PropertySource;
import org.springframework.context.support.PropertySourcesPlaceholderConfigurer;
import org.springframework.stereotype.Component; /**
*
*/
@Component
@PropertySource("classpath:properties/thread-pool.properties")
public class ThreadPoolConfig {
/**
* 核心线程个数
*/
@Value("${core.pool.size}")
private int corePoolSize;
/**
* 最大线程个数
*/
@Value("${max.pool.size}")
private int maxPoolSize;
/**
* 保持心跳时间
*/
@Value("${keep.alive.time}")
private int keeAliveTime;
/**
* 任务队列长度
*/
@Value("${task.queue.size}")
private int taskQueueSize;
/**
* 等待任务结束的时间
*/
@Value("${await.termination.time}")
private int awaitTerminationTime; /**
* 使用@value注解注入properties中的属性
* 1. 在类名上面使用 @PropertySource("classpath:*") 注解,*代表属性文件路径,可以指向多个配置文件路径
* 如果是多个配置文件,则是 @PropertySource({"classpath:*","classpath:*"....})
* 2. 在字段上直接使用@value注解
* 3. 注解内使用${core.pool.size} core.pool.size 代表属性文件里面的key
* 5. 需要新增 PropertySourcesPlaceholderConfigurer 的 bean
* 6. 在 PropertySourcesPlaceholderConfigurer 增加@bean注解,申明返回的是一个bean,否则会注入失败
*
*/ @Bean
public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
return new PropertySourcesPlaceholderConfigurer();
} public int getCorePoolSize() {
return corePoolSize;
} public void setCorePoolSize(int corePoolSize) {
this.corePoolSize = corePoolSize;
} public int getMaxPoolSize() {
return maxPoolSize;
} public void setMaxPoolSize(int maxPoolSize) {
this.maxPoolSize = maxPoolSize;
} public int getKeeAliveTime() {
return keeAliveTime;
} public void setKeeAliveTime(int keeAliveTime) {
this.keeAliveTime = keeAliveTime;
} public int getTaskQueueSize() {
return taskQueueSize;
} public void setTaskQueueSize(int taskQueueSize) {
this.taskQueueSize = taskQueueSize;
} public int getAwaitTerminationTime() {
return awaitTerminationTime;
} public void setAwaitTerminationTime(int awaitTerminationTime) {
this.awaitTerminationTime = awaitTerminationTime;
} @Override
public String toString() {
return new Gson().toJson(this);
}
}
这里注入了一个 PropertySourcesPlaceholderConfigurer bean，spring是通过 PropertySourcesPlaceholderConfigurer 的 locations 来查找属性文件，然后再根据注解将匹配的属性set进去，下面通过源码来了解注解可以进行一些什么操作。
public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerSupport implements EnvironmentAware {
/**
* {@value} is the name given to the {@link PropertySource} for the set of
* {@linkplain #mergeProperties() merged properties} supplied to this configurer.
*/
public static final String LOCAL_PROPERTIES_PROPERTY_SOURCE_NAME = "localProperties";
/**
* {@value} is the name given to the {@link PropertySource} that wraps the
* {@linkplain #setEnvironment environment} supplied to this configurer.
*/
public static final String ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME = "environmentProperties";
@Nullable
private MutablePropertySources propertySources;
@Nullable
private PropertySources appliedPropertySources;
@Nullable
private Environment environment;
下面代码省略。。。上面源码并没能说明为什么一定要返回这个bean，接下来看父类 PlaceholderConfigurerSupport 的源码：
/**
* Abstract base class for property resource configurers that resolve placeholders
* in bean definition property values. Implementations <em>pull</em> values from a
* properties file or other {@linkplain org.springframework.core.env.PropertySource
* property source} into bean definitions.
*
* <p>The default placeholder syntax follows the Ant / Log4J / JSP EL style:
*
* <pre class="code">${...}</pre>
*
* Example XML bean definition:
*
* <pre class="code">
* <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"/>
* <property name="driverClassName" value="${driver}"/>
* <property name="url" value="jdbc:${dbname}"/>
* </bean>
* </pre>
*
* Example properties file:
*
* <pre class="code">driver=com.mysql.jdbc.Driver
* dbname=mysql:mydb</pre>
*
* Annotated bean definitions may take advantage of property replacement using
* the {@link org.springframework.beans.factory.annotation.Value @Value} annotation:
*
* <pre class="code">@Value("${person.age}")</pre>
*
* Implementations check simple property values, lists, maps, props, and bean names
* in bean references. Furthermore, placeholder values can also cross-reference
* other placeholders, like:
*
* <pre class="code">rootPath=myrootdir
* subPath=${rootPath}/subdir</pre>
*
* In contrast to {@link PropertyOverrideConfigurer}, subclasses of this type allow
* filling in of explicit placeholders in bean definitions.
*
* <p>If a configurer cannot resolve a placeholder, a {@link BeanDefinitionStoreException}
* will be thrown. If you want to check against multiple properties files, specify multiple
* resources via the {@link #setLocations locations} property. You can also define multiple
* configurers, each with its <em>own</em> placeholder syntax. Use {@link
* #ignoreUnresolvablePlaceholders} to intentionally suppress throwing an exception if a
* placeholder cannot be resolved.
*
* <p>Default property values can be defined globally for each configurer instance
* via the {@link #setProperties properties} property, or on a property-by-property basis
* using the default value separator which is {@code ":"} by default and
* customizable via {@link #setValueSeparator(String)}.
*
* <p>Example XML property with default value:
*
* <pre class="code">
* <property name="url" value="jdbc:${dbname:defaultdb}"/>
* </pre>
*
* @author Chris Beams
* @author Juergen Hoeller
* @since 3.1
* @see PropertyPlaceholderConfigurer
* @see org.springframework.context.support.PropertySourcesPlaceholderConfigurer
*/
public abstract class PlaceholderConfigurerSupport extends PropertyResourceConfigurer
implements BeanNameAware, BeanFactoryAware { /** Default placeholder prefix: {@value} */
public static final String DEFAULT_PLACEHOLDER_PREFIX = "${"; /** Default placeholder suffix: {@value} */
public static final String DEFAULT_PLACEHOLDER_SUFFIX = "}"; /** Default value separator: {@value} */
public static final String DEFAULT_VALUE_SEPARATOR = ":"; /** Defaults to {@value #DEFAULT_PLACEHOLDER_PREFIX} */
protected String placeholderPrefix = DEFAULT_PLACEHOLDER_PREFIX; /** Defaults to {@value #DEFAULT_PLACEHOLDER_SUFFIX} */
protected String placeholderSuffix = DEFAULT_PLACEHOLDER_SUFFIX; /** Defaults to {@value #DEFAULT_VALUE_SEPARATOR} */
@Nullable
protected String valueSeparator = DEFAULT_VALUE_SEPARATOR; protected boolean trimValues = false; @Nullable
protected String nullValue; protected boolean ignoreUnresolvablePlaceholders = false; @Nullable
private String beanName; @Nullable
private BeanFactory beanFactory;
下面代码省略。。。
类注释说明了 PlaceholderConfigurerSupport 类所起的作用，以及替换了XML的哪些操作，其中就描述了注入的bean可以利用 @Value 注解进行属性替换：
* Annotated bean definitions may take advantage of property replacement using
* the {@link org.springframework.beans.factory.annotation.Value @Value} annotation:
*
* <pre class="code">@Value("${person.age}")</pre>
属性注释：
/** Default placeholder prefix: {@value} */
public static final String DEFAULT_PLACEHOLDER_PREFIX = "${";
/** Default placeholder suffix: {@value} */
public static final String DEFAULT_PLACEHOLDER_SUFFIX = "}";
/** Default value separator: {@value} */
public static final String DEFAULT_VALUE_SEPARATOR = ":";
/** Defaults to {@value #DEFAULT_PLACEHOLDER_PREFIX} */
protected String placeholderPrefix = DEFAULT_PLACEHOLDER_PREFIX;
/** Defaults to {@value #DEFAULT_PLACEHOLDER_SUFFIX} */
protected String placeholderSuffix = DEFAULT_PLACEHOLDER_SUFFIX;
/** Defaults to {@value #DEFAULT_VALUE_SEPARATOR} */
@Nullable
protected String valueSeparator = DEFAULT_VALUE_SEPARATOR;
protected boolean trimValues = false;
@Nullable
protected String nullValue;
protected boolean ignoreUnresolvablePlaceholders = false;从上面注解可以发现，使用的 默认前缀是：'${'，而后缀是：'}'，默认的分隔符是 ':'，但是set方法可以替换掉默认的分隔符，而 ignoreUnresolvablePlaceholders 默认为 false，表示会开启配置文件不存在，抛出异常的错误。
从上面就能看出这个bean所起的作用,就是将 @propertySource 注解的bean注入属性的作用，如果没有该bean，则不能解析${}符号。
接下来执行单元测试。
配置类代码：
package org.cellphone.web; import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration; /**
* 配置类——用来替换xml配置文件
*/
@Configuration
@ComponentScan("org.cellphone.config")
public class SpringConfig {
}
package org.cellphone.web; import org.cellphone.config.ThreadPoolConfig;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner; /**
* 纯注解方式整合Junit单元测试框架测试类
*/
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = { SpringConfig.class }) // 需要注意此处，将加载配置文件的注解换成加载配置类的注解
public class ThreadPoolConfigTest { private final Logger logger = LoggerFactory.getLogger(getClass()); @Autowired
private ThreadPoolConfig threadPoolConfig; @Test
public void testThreadPoolConfig() {
logger.info(threadPoolConfig.toString());
}
}
使用 @PropertySource 注解需要注意以下几个地方：
1. 使用注解需要将类申明为一个bean，可以使用 @Component 注解；
2. @PropertySource(value = "classpath:properties/config_userbean.properties", ignoreResourceNotFound = true) 表示注入配置文件，并且忽略配置文件不存在的异常；
3. 必须返回一个 PropertySourcesPlaceholderConfigurer 的bean，否则会不能识别@Value("${core.pool.size}") 注解中的 ${core.pool.size}指向的value，而会注入${core.pool.size}的字符串，返回 PropertySourcesPlaceholderConfigurer 的方法，使用 @Bean 注解，表示返回的是个bean。
在spring 4.0以后，spring增加了 @PropertySources 注解，可以使用多个 @PropertySource 注解，如下：
@PropertySources(
{
@PropertySource("classpath:properties/thread-pool.properties"),
@PropertySource("classpath:properties/mysql.properties")
}
)