Spring context:property-placeholder|极客教程

Spring context:property-placeholder教程展示了如何使用context:property-placeholder标记来外部化 Spring 应用中的属性。

Spring 是用于创建企业应用的流行 Java 应用框架。

Spring 的`context:property-placeholder`

context:property-placeholder标签用于外部化单独文件中的属性。它会自动配置PropertyPlaceholderConfigurer，从而替换${}占位符，这些占位符针对指定的属性文件（作为 Spring 资源位置）进行解析。

Spring `context:property-placeholder`示例

该应用使用context:property-placeholder来配置数据源的属性。

pom.xml
src
├───main
│   ├───java
│   │   └───com
│   │       └───zetcode
│   │               Application.java
│   └───resources
│           database.properties
│           logback.xml
│           my-beans.xml
└───test
    └───java

这是项目结构。

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
            http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.zetcode</groupId>
    <artifactId>propertyplaceholder</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <spring-version>5.1.3.RELEASE</spring-version>

    </properties>

    <dependencies>

        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.3</version>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>{spring-version}</version>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>{spring-version}</version>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-jdbc</artifactId>
            <version>${spring-version}</version>
        </dependency>        

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.6.0</version>
                <configuration>
                    <mainClass>com.zetcode.Application</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

在pom.xml文件中，我们具有基本的 Spring 依赖项spring-core，spring-context，spring-jdbc和日志记录logback-classic依赖项。

exec-maven-plugin用于在命令行上从 Maven 执行 Spring 应用。

resources/logback.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <logger name="org.springframework" level="ERROR"/>
    <logger name="com.zetcode" level="INFO"/>

    <appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <Pattern>%d{HH:mm:ss.SSS} %blue(%-5level) %magenta(%logger{36}) - %msg %n
            </Pattern>
        </encoder>
    </appender>

    <root>
        <level value="INFO" />
        <appender-ref ref="consoleAppender" />
    </root>
</configuration>

logback.xml是 Logback 日志库的配置文件。

resources/database.properties

db.url=jdbc:h2:mem:testdb
db.username=testuser
db.password=s$cret

这些值在database.properties文件中外部化。这种方法比将值直接放入 XML 文件中更为灵活。

resources/my-beans.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:context="http://www.springframework.org/schema/context"
        xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd">

    <context:property-placeholder location="classpath:database.properties"/>

    <bean id="dataSource" class="org.springframework.jdbc.datasource.SimpleDriverDataSource">
        <property name="url" value="{db.url}"></property>
        <property name="username" value="{db.username}"></property>
        <property name="password" value="${db.password}"></property>
    </bean>

</beans>

context:property-placeholder指定属性文件的位置；在我们的例子中，它是任何类路径目录中的database.properties文件。

<bean id="dataSource" class="org.springframework.jdbc.datasource.SimpleDriverDataSource">
    <property name="url" value="{db.url}"></property>
    <property name="username" value="{db.username}"></property>
    <property name="password" value="${db.password}"></property>
</bean>

定义了dataSource bean。它通过${}语法从属性文件中获取其值。

com/zetcode/Application.java

package com.zetcode;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.support.GenericXmlApplicationContext;
import org.springframework.jdbc.datasource.SimpleDriverDataSource;

public class Application {

    private static final Logger logger = LoggerFactory.getLogger(Application.class);

    public static void main(String[] args) {

        var ctx = new GenericXmlApplicationContext("my-beans.xml");

        var dataSource = (SimpleDriverDataSource) ctx.getBean("dataSource");

        logger.info("Url: {}", dataSource.getUrl());
        logger.info("User name: {}", dataSource.getUsername());
        logger.info("Password: {}", dataSource.getPassword());

        ctx.close();
    }
}

这是主要的应用类。它检索dataSource bean 并打印其属性。

$ mvn -q exec:java
11:27:43.790 INFO  com.zetcode.Application - Url: jdbc:h2:mem:testdb 
11:27:43.790 INFO  com.zetcode.Application - User name: testuser 
11:27:43.790 INFO  com.zetcode.Application - Password: s$cret