相信很多朋友都使用过如下代码进行可变数据的占位符替换。
<context:property-placeholder location="classpath:resources.properties" ignore-unresolvable="true"/>
最近使用maven profile + maven资源过滤 + spring3.2进行开发，如果不小心忘了maven的资源过滤，或者如遇到这种abc=${bcd} 这种循环引用时，且bcd不存在时，即：
如果代码中引用如abc：
@Value("${abc}")
private String abc;
且您的配置文件出现如下情况：
abc=${dce} ----->嵌套引用dce属性
dce=${none} --->此时none不存在
如果在spring3.1中，并配置了ignore-unresolvable，那么直接返回${none}，而不会在spring3.2中会遇到如下异常（spring3.1不会遇到问题）：
java.lang.StackOverflowError at java.security.AccessController.doPrivileged(Native Method) at com.sun.naming.internal.VersionHelper12.getJndiProperties(VersionHelper12.java:106) at com.sun.naming.internal.ResourceManager.getInitialEnvironment(ResourceManager.java:202) at javax.naming.InitialContext.init(InitialContext.java:238) at javax.naming.InitialContext.<init>(InitialContext.java:216) at org.springframework.jndi.JndiTemplate.createInitialContext(JndiTemplate.java:136) at org.springframework.jndi.JndiTemplate.getContext(JndiTemplate.java:103) at org.springframework.jndi.JndiTemplate.execute(JndiTemplate.java:85) at org.springframework.jndi.JndiTemplate.lookup(JndiTemplate.java:152) at org.springframework.jndi.JndiTemplate.lookup(JndiTemplate.java:178) at org.springframework.jndi.JndiLocatorSupport.lookup(JndiLocatorSupport.java:95) at org.springframework.jndi.JndiLocatorDelegate.lookup(JndiLocatorDelegate.java:38) at org.springframework.jndi.JndiLocatorSupport.lookup(JndiLocatorSupport.java:77) at org.springframework.jndi.JndiLocatorDelegate.lookup(JndiLocatorDelegate.java:33) at org.springframework.jndi.JndiPropertySource.getProperty(JndiPropertySource.java:82) at org.springframework.core.env.PropertySourcesPropertyResolver.getProperty(PropertySourcesPropertyResolver.java:73) at org.springframework.core.env.PropertySourcesPropertyResolver.getProperty(PropertySourcesPropertyResolver.java:59) at org.springframework.core.env.AbstractEnvironment.getProperty(AbstractEnvironment.java:427) at org.springframework.context.support.PropertySourcesPlaceholderConfigurer$1.getProperty(PropertySourcesPlaceholderConfigurer.java:131) at org.springframework.context.support.PropertySourcesPlaceholderConfigurer$1.getProperty(PropertySourcesPlaceholderConfigurer.java:128) at org.springframework.core.env.PropertySourcesPropertyResolver.getProperty(PropertySourcesPropertyResolver.java:73) at org.springframework.core.env.PropertySourcesPropertyResolver.getProperty(PropertySourcesPropertyResolver.java:59) at org.springframework.core.env.AbstractPropertyResolver$1.resolvePlaceholder(AbstractPropertyResolver.java:176) at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:146) at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:125)
其实就是死循环了，造成栈溢出。
源码分析：
1、<context:property-placeholder> 实际使用的是org.springframework.context.support.PropertySourcesPlaceholderConfigurer，其是一个BeanFactory后处理器（BeanFactoryPostProcessor）：
protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
final ConfigurablePropertyResolver propertyResolver) throws BeansException {
propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
propertyResolver.setValueSeparator(this.valueSeparator);
StringValueResolver valueResolver = new StringValueResolver() {
public String resolveStringValue(String strVal) {
String resolved = ignoreUnresolvablePlaceholders ?
propertyResolver.resolvePlaceholders(strVal) :
propertyResolver.resolveRequiredPlaceholders(strVal);
return (resolved.equals(nullValue) ? null : resolved);
}
};
doProcessProperties(beanFactoryToProcess, valueResolver);
}
1.1、此处根据ignoreUnresolvablePlaceholders(<context:property-placeholder>中的ignore-unresolvable)决定是否必须查找到占位符的值，
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
………………
this.processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
}
1.2、最后一行直接委托给了processProperties方法 并传入了PropertySourcesPropertyResolver，这个是核心，问题就出在这；
1.3、然后不管propertyResolver.propertyResolver.resolvePlaceholders还是propertyResolver.resolveRequiredPlaceholders都会调用：
return doResolvePlaceholders(text, strictHelper/nonStrictHelper);
只是此处一个使用严格的PropertyPlaceholderHelper 另一个是不严格的PropertyPlaceholderHelper；
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
return helper.replacePlaceholders(text, new PlaceholderResolver() {
public String resolvePlaceholder(String placeholderName) {
return getProperty(placeholderName);
}
});
}
1.4、 此时调用helper的
public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
Assert.notNull(value, "Argument 'value' must not be null.");
return parseStringValue(value, placeholderResolver, new HashSet<String>());
}
1.5、parseStringValue方法：
如果发现如abc=${dce}，那么会接着调用如下代码接着循环找值
String propVal = placeholderResolver.resolvePlaceholder(placeholder);
1.6、最终会调用1.3中的getProperty方法去获取值，即此时key是dce：
public String getProperty(String key) {
if (logger.isTraceEnabled()) {
logger.trace(format("getProperty(\"%s\") (implicit targetType [String])", key));
}
return this.getProperty(key, String.class);
}
最终会调用：
if (String.class.equals(valueType)) {
value = this.resolveNestedPlaceholders((String) value);
}
1.7、到了resolveNestedPlaceholders
protected String resolveNestedPlaceholders(String value) {
return this.ignoreUnresolvableNestedPlaceholders ?
this.resolvePlaceholders(value) :
this.resolveRequiredPlaceholders(value);
}
问题出现了：如果此处的ignoreUnresolvableNestedPlaceholders为false，那么接着又去了resolveRequiredPlaceholders，此是就会死循环问题，spring3.2没有考虑这个问题，所以会造成java.lang.StackOverflowError。
解决方案：
1、spring考虑此循环引用的问题
2、在org.springframework.context.support.PropertySourcesPlaceholderConfigurer中在new PropertySourcesPropertyResolver(this.propertySources)后调用其setIgnoreUnresolvableNestedPlaceholders(ignoreUnresolvablePlaceholders)，把ignoreUnresolvablePlaceholders传入即可。
到此分析结束。