1. PropertyPlaceholderConfigurer是个bean工厂后置处理器的实现，也就是 BeanFactoryPostProcessor接口的一个实现。PropertyPlaceholderConfigurer可以将上下文（配置文 件）中的属性值放在另一个单独的标准java Properties文件中去。在XML文件中用${key}替换指定的properties文件中的值。这样的话，只需要对properties文件进 行修改，而不用对xml配置文件进行修改。

2.在Spring中，使用PropertyPlaceholderConfigurer可以在XML配置文件中加入外部属性文件，当然也可以指定外部文件的编码，如：

 <bean id="propertyConfigurer"class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">

    <property name="location">

      <value>conf/jdbc.properties</value>

    </property>

     <property name="fileEncoding">

       <value>UTF-8</value>

     </property>

 </bean>

3.PropertyPlaceholderConfigurer起的作用就是将占位符指向的数据库配置信息放在bean中定义的工具。

4.查看源代码，可以发现，locations属性定义在PropertyPlaceholderConfigurer的祖父类 PropertiesLoaderSupport中，而location只有 setter方法。类似于这样的配置，在spring的源程序中很常见的。

PropertyPlaceholderConfigurer如果在指定的Properties文件中找不到你想使用的属性，它还会在Java的System类属性中查找。

我们可以通过System.setProperty(key, value)或者java中通过-Dnamevalue来给Spring配置文件传递参数。

实例：

<bean class="com.slp.util.SpringContextHolder"/>

        <bean id="propertyConfigurer"

        class="com.slp.util.CustomizedPropertyConfigurer">

        <property name="locations">

            <list>

                <value>classpath:conf/jdbc.properties</value>                <value>classpath:conf/config.properties</value>                <value>classpath:conf/server.properties</value>

            </list>

        </property>

        <property name="fileEncoding" value="UTF-8"/>

    </bean>

package com.slp.util;

import org.springframework.beans.BeansException;

import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;

import org.springframework.beans.factory.config.PropertyPlaceholderConfigurer;

import java.util.HashMap;

import java.util.Map;

import java.util.Properties;

public class CustomizedPropertyConfigurer extends PropertyPlaceholderConfigurer {

    private static Map<String,String> ctxPropMap;

    @Override

    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props) throws BeansException {

        super.processProperties(beanFactoryToProcess, props);

        ctxPropMap = new HashMap<>();

        for (Object key : props.keySet()){

            String keyStr = key.toString();

            String value = String.valueOf(props.get(keyStr));

            ctxPropMap.put(keyStr,value);

        }

    }

    public static String getCtxProp(String name) {

        return ctxPropMap.get(name);

    }

    public static Map<String, String> getCtxPropMap() {

        return ctxPropMap;

    }

}

//使用的时候如下即可

CustomizedPropertyConfigurer.getCtxProp("xxx")

org.springframework.beans.factory.config

Class PropertyPlaceholderConfigurer

java.lang.Object-->

org.springframework.core.io.support.PropertiesLoaderSupport-->

org.springframework.beans.factory.config.PropertyResourceConfigurer-->

org.springframework.beans.factory.config.PlaceholderConfigurerSupport-->

org.springframework.beans.factory.config.PropertyPlaceholderConfigurer

以上即是PropertyPlaceholderConfigurer类的继承关系，

All Implemented Interfaces:所实现的接口

Aware, BeanFactoryAware, BeanNameAware, BeanFactoryPostProcessor, Ordered, PriorityOrdered

Direct Known Subclasses:(直接的已知的子类)

PreferencesPlaceholderConfigurer

直接的子类：PreferencesPlaceholderConfigurer

public class PropertyPlaceholderConfigurer

extends PlaceholderConfigurerSupport

PlaceholderConfigurerSupport subclass that resolves ${...} placeholders against local properties and/or system properties and environment variables.

PlaceholderConfigurerSupport 的子类是用于解决${}的占位符，比如本地变量 或者系统变量或者环境变量等。

As of Spring 3.1, PropertySourcesPlaceholderConfigurer should be used preferentially over this implementation; it is more flexible through taking advantage of the Environment and PropertySource mechanisms also made available in Spring 3.1.

从spring3.1开始，propertysourcesplaceholderconfigurer应该在这些实现中优先使用，通过利用Environment 和PropertySource机制使得其更灵活。

PropertyPlaceholderConfigurer is still appropriate(合适的) for use when:

• the spring-context module is not available (i.e., one is using Spring's BeanFactory API as opposed to ApplicationContext).
• existing configuration makes use of the "systemPropertiesMode" and/or "systemPropertiesModeName" properties. Users are encouraged to move away from using these settings, and rather configure property source search order through the container's Environment; however, exact preservation of functionality may be maintained by continuing to use PropertyPlaceholderConfigurer.

Prior to Spring 3.1, the <context:property-placeholder/> namespace element registered an instance of PropertyPlaceholderConfigurer. It will still do so if using the spring-context-3.0.xsd definition of the namespace. That is, you can preserve registration of PropertyPlaceholderConfigurer through the namespace, even if using Spring 3.1; simply do not update your xsi:schemaLocation and continue using the 3.0 XSD.

在spring3.1之前<context:property-placeholder/> 命名空间注册了一个PropertyPlaceholderConfigurer的实例，如果你使用spring-context-3.0.xsd 之前依然是可以使用。

示例：例如一些配置文件中的常量为了简化不用频繁的更改参数配置信息，spring3.0中提供了一种简单的方式context:property-placeholder 元素。

只需要在spring的配置文件里添加一句：<context:property-placeholder location=””/>即可。这里的location值为参数配置文件的位置，参数配置文件与常用的参数配置文件相同，即键值对的形式。

PropertyPlaceholderConfiguration内置的功能非常丰富，如果未找到${xxx}中定义的xxx键，他还会去JVM系统属性(System.getProperty())和环境变量(System.getenv())中寻找，通过启用systemPropertiesMode和searchSystemEnviroment属性，可以控制这一行为。

Since:

02.10.2003

Author:

Juergen Hoeller, Chris Beams

See Also:

setSystemPropertiesModeName(java.lang.String), PlaceholderConfigurerSupport, PropertyOverrideConfigurer, PropertySourcesPlaceholderConfigurer

Field Summary

• DEFAULT_PLACEHOLDER_PREFIX, DEFAULT_PLACEHOLDER_SUFFIX, DEFAULT_VALUE_SEPARATOR, ignoreUnresolvablePlaceholders, nullValue, placeholderPrefix, placeholderSuffix, trimValues, valueSeparator
• localOverride, localProperties, logger
• HIGHEST_PRECEDENCE, LOWEST_PRECEDENCE

Fields inherited from class org.springframework.beans.factory.config.PlaceholderConfigurerSupport

Fields inherited from class org.springframework.core.io.support.PropertiesLoaderSupport

Fields inherited from interface org.springframework.core.Ordered

Constructor Summary

Method Summary

• doProcessProperties, setBeanFactory, setBeanName, setIgnoreUnresolvablePlaceholders, setNullValue, setPlaceholderPrefix, setPlaceholderSuffix, setTrimValues, setValueSeparator
• convertProperties, convertProperty, convertPropertyValue, getOrder, postProcessBeanFactory, setOrder
• loadProperties, mergeProperties, setFileEncoding, setIgnoreResourceNotFound, setLocalOverride, setLocation, setLocations, setProperties, setPropertiesArray, setPropertiesPersister
• clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from class org.springframework.beans.factory.config.PlaceholderConfigurerSupport

Methods inherited from class org.springframework.beans.factory.config.PropertyResourceConfigurer

Methods inherited from class org.springframework.core.io.support.PropertiesLoaderSupport

Methods inherited from class java.lang.Object

Field Detail

SYSTEM_PROPERTIES_MODE_NEVER

public static final int SYSTEM_PROPERTIES_MODE_NEVER

Never check system properties.

See Also:

Constant Field Values

SYSTEM_PROPERTIES_MODE_FALLBACK

public static final int SYSTEM_PROPERTIES_MODE_FALLBACK

Check system properties if not resolvable in the specified properties. This is the default.

See Also:

Constant Field Values

SYSTEM_PROPERTIES_MODE_OVERRIDE

public static final int SYSTEM_PROPERTIES_MODE_OVERRIDE

Check system properties first, before trying the specified properties. This allows system properties to override any other property source.

See Also:

Constant Field Values

Constructor Detail

PropertyPlaceholderConfigurer

public PropertyPlaceholderConfigurer()

Method Detail

setSystemPropertiesModeName

public void setSystemPropertiesModeName(String constantName)

throws IllegalArgumentException

Set the system property mode by the name of the corresponding constant, e.g. "SYSTEM_PROPERTIES_MODE_OVERRIDE".

Parameters:

constantName - name of the constant

Throws:

IllegalArgumentException - if an invalid constant was specified

See Also:

setSystemPropertiesMode(int)

setSystemPropertiesMode

public void setSystemPropertiesMode(int systemPropertiesMode)

Set how to check system properties: as fallback, as override, or never. For example, will resolve ${user.dir} to the "user.dir" system property.

The default is "fallback": If not being able to resolve a placeholder with the specified properties, a system property will be tried. "override" will check for a system property first, before trying the specified properties. "never" will not check system properties at all.

See Also:

SYSTEM_PROPERTIES_MODE_NEVER, SYSTEM_PROPERTIES_MODE_FALLBACK, SYSTEM_PROPERTIES_MODE_OVERRIDE, setSystemPropertiesModeName(java.lang.String)

setSearchSystemEnvironment

public void setSearchSystemEnvironment(boolean searchSystemEnvironment)

Set whether to search for a matching system environment variable if no matching system property has been found. Only applied when "systemPropertyMode" is active (i.e. "fallback" or "override"), right after checking JVM system properties.

Default is "true". Switch this setting off to never resolve placeholders against system environment variables. Note that it is generally recommended to pass external values in as JVM system properties: This can easily be achieved in a startup script, even for existing environment variables.

NOTE: Access to environment variables does not work on the Sun VM 1.4, where the corresponding System.getenv(java.lang.String) support was disabled - before it eventually got re-enabled for the Sun VM 1.5. Please upgrade to 1.5 (or higher) if you intend to rely on the environment variable support.

See Also:

setSystemPropertiesMode(int), System.getProperty(String), System.getenv(String)

resolvePlaceholder

protected String resolvePlaceholder(String placeholder,Properties props,int systemPropertiesMode)

Resolve the given placeholder using the given properties, performing a system properties check according to the given mode.

The default implementation delegates to resolvePlaceholder (placeholder, props) before/after the system properties check.

Subclasses can override this for custom resolution strategies, including customized points for the system properties check.

Parameters:

placeholder - the placeholder to resolve

props - the merged properties of this configurer

systemPropertiesMode - the system properties mode, according to the constants in this class

Returns:

the resolved value, of null if none

See Also:

setSystemPropertiesMode(int), System.getProperty(java.lang.String), resolvePlaceholder(String, java.util.Properties)

resolvePlaceholder

protected String resolvePlaceholder(String placeholder, Properties props)

Resolve the given placeholder using the given properties. The default implementation simply checks for a corresponding property key.

Subclasses can override this for customized placeholder-to-key mappings or custom resolution strategies, possibly just using the given properties as fallback.

Note that system properties will still be checked before respectively after this method is invoked, according to the system properties mode.

Parameters:

placeholder - the placeholder to resolve

props - the merged properties of this configurer

Returns:

the resolved value, of null if none

See Also:

setSystemPropertiesMode(int)

resolveSystemProperty

protected String resolveSystemProperty(String key)

Resolve the given key as JVM system property, and optionally also as system environment variable if no matching system property has been found.

Parameters:

key - the placeholder to resolve as system property key

Returns:

the system property value, or null if not found

See Also:

setSearchSystemEnvironment(boolean), System.getProperty(String), System.getenv(String)

processProperties

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,Properties props)throws BeansException

Visit each bean definition in the given bean factory and attempt to replace ${...} property placeholders with values from the given properties.

Specified by:

processProperties in class PropertyResourceConfigurer

Parameters:

beanFactoryToProcess - the BeanFactory used by the application context

props - the Properties to apply

Throws:

BeansException - in case of errors