spring学习（一）属性管理PropertySource类

一、PropertySource：用于存放key-value对象的抽象，子类需要实现getProperty(String name)返回对应的Value方法，其中value可以是任何类型不局限在字符串

注:PropertySource里的属性name和source都是final的。初始化后不能修改

其中named(String name)是用来判断数组里是否包括当前name的方法，spring给的例子如下

@Test
	public void testSource() {
		List> sources = new ArrayList>();
		sources.add(new MapPropertySource("sourceA", new HashMap()));
		sources.add(new MapPropertySource("sourceB", new HashMap()));
		Assert.assertEquals(true, sources.contains(PropertySource.named("sourceA")));
		Assert.assertEquals(true, sources.contains(PropertySource.named("sourceB")));
		Assert.assertEquals(false, sources.contains(PropertySource.named("sourceC")));
 	}

PropertySource.named的方法实现

public static PropertySource named(String name) {
		return new ComparisonPropertySource(name);
	}

其中ComparisonPropertySource只对List有用，只继承了getName方法。其中方法都会抛出异常

ComparisonPropertySource instances are for collection comparison use only

对应子类

EnumerablePropertySource：增加了一个方法用于返回所有name值getPropertyNames，同时重写的containsProperty方法，通过getPropertyNames返回的key值进行判断，有助于提升性能

MapPropertySource：其中的source是以Map形式存放的

重写了getProperty和getPropertyNames

PropertiesPropertySoruce：同MapPropertySource,只是构造函数的参数不同

二、PropertySources：用于存放PropertySource的集合

MutablePropertySources：用linkList实现PropertySources，可以方便向List链中首位、末位、中间位置增加或替换或删除一个key-value属性值

每次增加或替换时，都会判断这个PropertySource是否存在，如果存在，先删除。保证整个List中name的唯一

刚开始看源代码，MapPropertySource和MutablePropertySources总是容易弄混。慢慢看了一些应用的源代码。比如spring在预启动时，会加载系统的环境变量。StandardEnvironment代码如下

	@Override
	protected void customizePropertySources(MutablePropertySources propertySources) {
		propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
		propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
	}

父类AbstractEnvironment代码如下

public AbstractEnvironment() {
	customizePropertySources(this.propertySources);
}

此时的环境变量结构如下图表示,而StandardEnvironment.getProperty(key)是需要遍历MutablePropertySources里面的所有PropertySource，并查看是否有存在的key值

三,PropertySource应用

spring在加载通过xml文件配置的bean文件，可以在文件路径地址中写入一些系统参数信息，比如例子如下（可能配置的没什么意义）

public class TestPropertyEditorRegistrar {

    private ClassPathXmlApplicationContext context;

    @Before
    public void setUp() throws Exception {
        context = new ClassPathXmlApplicationContext(new String[] { "classpath*:spring/${java.vm.version}/propertyEditor.xml" });
    }

｝

其中的${java.vm.version}会在正式加载之前通过StandardEnvironment替换成相应的变量名称

1)在ClassPathXmlApplicationContext的父类中AbstractRefreshableConfigApplicationContext中的setConfigLocations(String ... locations)有如下方法

	protected void setConfigLocations(String[] locations) {
		if (locations != null) {
			Assert.notNullElements(locations, "Config locations must not be null");
			configLocations = new String[locations.length];
			for (int i = 0; i < locations.length; i++) {
				configLocations[i] = resolvePath(locations[i]).trim();
			}
		} else {
			this.configLocations = null;
		}
	}
	
	protected String resolvePath(String path) {
		//调用PropertyResolver.resolveRequiredPlaceholders的方法，
		//其中getEnvironment得到的对象是StandardEnvironment，而这个方法是由父类AbstractEnvironment中实现
		return getEnvironment().resolveRequiredPlaceholders(path);
	}

2)AbstractEnvironment实现了Envirionment和PropertyResolver接口，但对于PropertyResolver的接口中的方法都是通过PropertySourcesPropertyResolver类实现的，PropertySourcesPropertyResolver这个同PropertyResolver的一个子类

	private final MutablePropertySources propertySources = new MutablePropertySources();
	
	private final ConfigurablePropertyResolver propertyResolver = new PropertySourcesPropertyResolver(propertySources);

	@Override
	public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
		//调用接口PropertyResolver进行具体的解析字串.类PropertySourcesPropertyResolver的对象生成时会传递一个MutablePropertySources对象
		//StandardEnvironment的的实现传递了2个PropertySource,(systemEnvironment,systemProperties)
		//最终调用的是AbstractPropertyResolver.resolveRequiredPlaceholders方法
		return this.propertyResolver.resolveRequiredPlaceholders(text);
	}

3)AbstractPropertyResolver.resolveRequiredPlaceholders的关键实现类是PropertyPlaceholderHelper.replacePlaceholders方法

public String resolveRequiredPlaceholders(String value) {
		if (strictHelper == null) {
			strictHelper = createPlaceholderHelper(true);
		}
		return doResolvePlaceholders(value, strictHelper);
}
/**
* 定义一个PropertyPlaceholderHelper，并传参数用于判断是否忽略不能解析的变量
*/
public PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvableNestedPlaceholders) {
	return new PropertyPlaceholderHelper(placeholderPrefix, placeholderSuffix, valueSeparator, ignoreUnresolvableNestedPlaceholders);
}

	private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
		//直接调用helper.parseStringValue方法，这个方法也得spring用来解析字符串中所有替换变量实现类,
		//这个方法的实现的可以单独说明,主要功能是替换text中的${xxxx}指导xxxx替换成getProperty(placeholderName)取到的变量值
		return helper.parseStringValue(text, new PlaceHolderResolver() {
			@Override
			public String resolvePlaceholder(String placeholderName) {
				//这个方法是由具体的PropertyResolver的子类实现的
				return getProperty(placeholderName);
			}
		});
	}

4)PropertyResolver.getProperty(name)的实现，这里是由PropertySourcePropertyResolver类实现,并支持嵌套替换

@Override
	public String getProperty(String key) {
		if (logger.isTraceEnabled()) {
			logger.trace(format("getProperty(\"%s\")(implicit targetType [String])", key));
		}
		return getProperty(key, String.class);
	}
	public  T getProperty(String key, Class requiredType) {
		boolean debugEnabled = logger.isDebugEnabled();
		if (logger.isTraceEnabled()) {
			logger.trace(format("getProperty(\"%s\", %s)", key, requiredType.getSimpleName()));
		}
		for (PropertySource propertySource : propertySources) {
			if (debugEnabled) {
				logger.debug(format("Searching for key '%s' in [%s]", key, propertySource.getName()));
			}
			Object value;
			if ((value = propertySource.getProperty(key)) != null) {
				Class valueType = value.getClass();
				//如果类型是strng，可以嵌套解析
				if (String.class.equals(valueType)) {
					value = this.resolveNestedPlaceholders((String)value);
				}
				if (debugEnabled) {
					logger.debug(
							format("Found key '%s' in [%s] with type [%s] and value '%s'",
									key, propertySource.getName(), valueType.getSimpleName(), value));
				}
				return (T)value;
			}
		}
		if (debugEnabled) {
			logger.debug(format("Could not find key '%s' in any property source. Returning [null]", key));
		}
		return null;
	}