我最近遇到了如何在环境中注册自定义属性源的问题。我的具体问题是我有一个带有 Spring 配置的库，我想将它导入到 Spring 应用程序上下文中，并且它需要自定义属性源。但是，我不一定可以控制创建应用程序上下文的所有位置。因此，我不想使用推荐的 ApplicationContextInitializer 或 register-before-refresh 机制来注册自定义属性源。
我发现真正令人沮丧的是，使用旧的 PropertyPlaceholderConfigurer，很容易在 Spring 配置中完全继承和自定义配置器。相反，要自定义属性源，我们被告知我们必须在 Spring 配置本身中而不是在初始化应用程序上下文之前进行。
经过一些研究和反复试验，我发现 可以从 Spring 配置内部注册自定义属性源，但您必须小心如何操作。在上下文中执行任何 PropertySourcesPlaceholderConfigurers 之前，需要注册源。您可以通过将源注册设置为具有 PriorityOrdered 的 BeanFactoryPostProcessor 和比使用源的 PropertySourcesPlaceholderConfigurer 更高优先级的顺序来做到这一点。
我写了这个类，它完成了这项工作：
package example;
import java.io.IOException;
import java.util.Properties;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.core.Ordered;
import org.springframework.core.PriorityOrdered;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.core.env.PropertiesPropertySource;
import org.springframework.core.env.PropertySource;
import org.springframework.core.io.support.PropertiesLoaderSupport;
/**
* This is an abstract base class that can be extended by any class that wishes
* to become a custom property source in the Spring context.
* <p>
* This extends from the standard Spring class PropertiesLoaderSupport, which
* contains properties that specify property resource locations, plus methods
* for loading properties from specified resources. These are all available to
* be used from the Spring configuration, and by subclasses of this class.
* <p>
* This also implements a number of Spring flag interfaces, all of which are
* required to maneuver instances of this class into a position where they can
* register their property sources BEFORE PropertySourcesPlaceholderConfigurer
* executes to substitute variables in the Spring configuration:
* <ul>
* <li>BeanFactoryPostProcessor - Guarantees that this bean will be instantiated
* before other beans in the context. It also puts it in the same phase as
* PropertySourcesPlaceholderConfigurer, which is also a BFPP. The
* postProcessBeanFactory method is used to register the property source.</li>
* <li>PriorityOrdered - Allows the bean priority to be specified relative to
* PropertySourcesPlaceholderConfigurer so that this bean can be executed first.
* </li>
* <li>ApplicationContextAware - Provides access to the application context and
* its environment so that the created property source can be registered.</li>
* </ul>
* <p>
* The Spring configuration for subclasses should contain the following
* properties:
* <ul>
* <li>propertySourceName - The name of the property source this will register.</li>
* <li>location(s) - The location from which properties will be loaded.</li>
* <li>addBeforeSourceName (optional) - If specified, the resulting property
* source will be added before the given property source name, and will
* therefore take precedence.</li>
* <li>order (optional) - The order in which this source should be executed
* relative to other BeanFactoryPostProcessors. This should be used in
* conjunction with addBeforeName so that if property source factory "psfa"
* needs to register its property source before the one from "psfb", "psfa"
* executes AFTER "psfb".
* </ul>
*
* @author rjsmith2
*
*/
public abstract class AbstractPropertySourceFactory extends
PropertiesLoaderSupport implements ApplicationContextAware,
PriorityOrdered, BeanFactoryPostProcessor {
// Default order will be barely higher than the default for
// PropertySourcesPlaceholderConfigurer.
private int order = Ordered.LOWEST_PRECEDENCE - 1;
private String propertySourceName;
private String addBeforeSourceName;
private ApplicationContext applicationContext;
private MutablePropertySources getPropertySources() {
final Environment env = applicationContext.getEnvironment();
if (!(env instanceof ConfigurableEnvironment)) {
throw new IllegalStateException(
"Cannot get environment for Spring application context");
}
return ((ConfigurableEnvironment) env).getPropertySources();
}
public int getOrder() {
return order;
}
public void setOrder(int order) {
this.order = order;
}
public String getPropertySourceName() {
return propertySourceName;
}
public void setPropertySourceName(String propertySourceName) {
this.propertySourceName = propertySourceName;
}
public String getAddBeforeSourceName() {
return addBeforeSourceName;
}
public void setAddBeforeSourceName(String addBeforeSourceName) {
this.addBeforeSourceName = addBeforeSourceName;
}
public void setApplicationContext(ApplicationContext applicationContext) {
this.applicationContext = applicationContext;
}
/**
* Subclasses can override this method to perform adjustments on the
* properties after they are read.
* <p>
* This should be done by getting, adding, removing, and updating properties
* as needed.
*
* @param props
* properties to adjust
*/
protected void convertProperties(Properties props) {
// Override in subclass to perform conversions.
}
/**
* Creates a property source from the specified locations.
*
* @return PropertiesPropertySource instance containing the read properties
* @throws IOException
* if properties cannot be read
*/
protected PropertySource<?> createPropertySource() throws IOException {
if (propertySourceName == null) {
throw new IllegalStateException("No property source name specified");
}
// Load the properties file (or files) from specified locations.
final Properties props = new Properties();
loadProperties(props);
// Convert properties as required.
convertProperties(props);
// Convert to property source.
final PropertiesPropertySource source = new PropertiesPropertySource(
propertySourceName, props);
return source;
}
@Override
public void postProcessBeanFactory(
ConfigurableListableBeanFactory beanFactory) throws BeansException {
try {
// Create the property source, and get its desired position in
// the list of sources.
if (logger.isDebugEnabled()) {
logger.debug("Creating property source [" + propertySourceName
+ "]");
}
final PropertySource<?> source = createPropertySource();
// Register the property source.
final MutablePropertySources sources = getPropertySources();
if (addBeforeSourceName != null) {
if (sources.contains(addBeforeSourceName)) {
if (logger.isDebugEnabled()) {
logger.debug("Adding property source ["
+ propertySourceName + "] before ["
+ addBeforeSourceName + "]");
}
sources.addBefore(addBeforeSourceName, source);
} else {
logger.warn("Property source [" + propertySourceName
+ "] cannot be added before non-existent source ["
+ addBeforeSourceName + "] - adding at the end");
sources.addLast(source);
}
} else {
if (logger.isDebugEnabled()) {
logger.debug("Adding property source ["
+ propertySourceName + "] at the end");
}
sources.addLast(source);
}
} catch (Exception e) {
throw new BeanInitializationException(
"Failed to register property source", e);
}
}
}
这里需要注意的是，这个属性源工厂类的默认顺序比PropertySourcesPlaceholderConfigurer的默认顺序要高。
此外，属性源的注册发生在 postProcessBeanFactory 中，这意味着它将以相对于 PropertySourcesPlaceholderConfigurer 的正确顺序执行。我发现 InitializingBean 和 afterPropertiesSet 不尊重 order 参数的困难方式，我放弃了这种方法，因为它是错误的和多余的。
最后，因为这是一个 BeanFactoryPostProcessor，所以尝试以依赖关系的方式连接很多是一个坏主意。因此，该类直接通过应用程序上下文访问环境，应用程序上下文是使用 ApplicationContextAware 获得的。
在我的例子中，我需要属性源来解密密码属性，我使用以下子类来实现：
package example;
import java.util.Properties;
/**
* This is a property source factory that creates a property source that can
* process properties for substituting into a Spring configuration.
* <p>
* The only thing that distinguishes this from a normal Spring property source
* is that it decrypts encrypted passwords.
*
* @author rjsmith2
*
*/
public class PasswordPropertySourceFactory extends
AbstractPropertySourceFactory {
private static final PasswordHelper passwordHelper = new PasswordHelper();
private String[] passwordProperties;
public String[] getPasswordProperties() {
return passwordProperties;
}
public void setPasswordProperties(String[] passwordProperties) {
this.passwordProperties = passwordProperties;
}
public void setPasswordProperty(String passwordProperty) {
this.passwordProperties = new String[] { passwordProperty };
}
@Override
protected void convertProperties(Properties props) {
// Adjust password fields by decrypting them.
if (passwordProperties != null) {
for (String propName : passwordProperties) {
final String propValue = props.getProperty(propName);
if (propValue != null) {
final String plaintext = passwordHelper
.decryptString(propValue);
props.setProperty(propName, plaintext);
}
}
}
}
}
最后，我在我的 Spring 配置中指定了属性源工厂：
<!-- Enable property resolution via PropertySourcesPlaceholderConfigurer.
The order has to be larger than the ones used by custom property sources
so that those property sources are registered before any placeholders
are substituted. -->
<context:property-placeholder order="1000" ignore-unresolvable="true" />
<!-- Register a custom property source that reads DB properties, and
decrypts the database password. -->
<bean class="example.PasswordPropertySourceFactory">
<property name="propertySourceName" value="DBPropertySource" />
<property name="location" value="classpath:db.properties" />
<property name="passwordProperty" value="db.password" />
<property name="ignoreResourceNotFound" value="true" />
<!-- Order must be lower than on property-placeholder element. -->
<property name="order" value="100" />
</bean>
说实话，使用 PropertySourcesPlaceholderConfigurer 和 AbstractPropertySourceFactory 中的默认顺序，可能甚至不需要在 Spring 配置中指定顺序。
尽管如此，这是可行的，并且它不需要对应用程序上下文初始化进行任何摆弄。