spring之通过PropertyPlaceholderHelper替换占位符

痛定思痛。 2022-11-16 13:58 289阅读 0赞

## 写在前面 ##

本文少量涉及源码分析，对源码感兴趣的朋友可以参考[这里][Link 1]。

## 1：替换单个key ##

### 1.1：测试代码 ###

@Test
    public void testReplaceSinglePlaceholder() { 
        PropertyPlaceholderHelper helper = new PropertyPlaceholderHelper("${", "}");
        String text = "foo=${foo}";
        System.out.println("替换前的值是：" + text);
        Properties props = new Properties();
        props.setProperty("foo", "bar");
        String replacedResult = helper.replacePlaceholders(text, props);
        System.out.println("替换后的值是：" + replacedResult);
    }

### 1.2：运行 ###

替换前的值是：foo=${ foo}
    替换后的值是：foo=bar

## 2：替换多个key ##

### 2.1：测试代码 ###

@Test
    public void testReplaceMultiPlaceholder() { 
        PropertyPlaceholderHelper helper = new PropertyPlaceholderHelper("${", "}");
        String text = "foo=${foo},bar=${bar}";
        System.out.println("替换前的值是：" + text);
        Properties props = new Properties();
        props.setProperty("foo", "bar");
        props.setProperty("bar", "baz");
        String replacedResult = helper.replacePlaceholders(text, props);
        System.out.println("替换后的值是：" + replacedResult);
    }

### 2.2：运行 ###

替换前的值是：foo=${foo},bar=${bar}
    替换后的值是：foo=bar,bar=baz

## 3：替换递归属性 ##

### 3.1：测试代码 ###

@Test
    public void testRecurseInProperty() { 
        PropertyPlaceholderHelper helper = new PropertyPlaceholderHelper("${", "}");
        String text = "foo=${bar}";
        System.out.println("替换前的值是：" + text);
        Properties props = new Properties();
        props.setProperty("bar", "${baz}");
        props.setProperty("baz", "bar");
        String replacedResult = helper.replacePlaceholders(text, props);
        System.out.println("替换后的值是：" + replacedResult);
    }

### 3.2：运行 ###

替换前的值是：foo=${bar}
    替换后的值是：foo=bar

## 4：自定义替换占位符数据源 ##

正常是提供一个`java.util.Properties`来作为数据源，我们也可以通过`org.springframework.util.PropertyPlaceholderHelper.PlaceholderResolver`接口来自定义数据源。

### 4.1：测试代码 ###

@Test
    public void replaceWithSelfDefineSource() { 
        PropertyPlaceholderHelper helper = new PropertyPlaceholderHelper("${", "}");
        String text = "foo=${foo},bar=${bar}";
        System.out.println("替换前的值是：" + text);
        Properties props = new Properties();
        props.setProperty("foo", "bar");
        props.setProperty("bar", "baz");
        // public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver)
        String replacedResult = helper.replacePlaceholders(text, (placeholderName) -> queryWithWithMySource(placeholderName));
        System.out.println("替换后的值是：" + replacedResult);
    }
    
    private String queryWithWithMySource(String placeholderName) { 
        String result = "";
        if ("foo".equals(placeholderName)) { 
            result = "bar_selfsource";
        } else if ("bar".equals(placeholderName)) { 
            result = "baz_selfsource";
        }
        return result;
    }

### 4.2：运行 ###

替换前的值是：foo=${ foo},bar=${ bar}
    替换后的值是：foo=bar_selfsource,bar=baz_selfsource

## 5：StringValueResolver ##

`org.springframework.util.StringValueResolver`是一个函数接口，源码如下：

@FunctionalInterface
    public interface StringValueResolver { 
    	// 解析字符串值，子类，实现该方法，使用自己的策略来解析，如占位符的替换
    	@Nullable
    	String resolveStringValue(String strVal);
    }

用来定义字符串的解析策略，其实就是将一个字符串转换为另外一个字符串，常见的如占位符的替换，如spring中其子类`org.springframework.beans.factory.config.PropertyPlaceholderConfigurer.PlaceholderResolvingStringValueResolver`，我们看下该子类源码：

org.springframework.beans.factory.config.PropertyPlaceholderConfigurer.PlaceholderResolvingStringValueResolver
    private class PlaceholderResolvingStringValueResolver implements StringValueResolver { 
    	// 执行替换真正使用的类
    	privatefinal PropertyPlaceholderHelper helper;
    	// 提供数据源的类
    	private final PlaceholderResolver resolver;
    
    	public PlaceholderResolvingStringValueResolver(Properties props) { 
    		this.helper = new PropertyPlaceholderHelper(
    				placeholderPrefix, placeholderSuffix, valueSeparator, ignoreUnresolvablePlaceholders);
    		this.resolver = new PropertyPlaceholderConfigurerResolver(props);
    	}
    
    	@Override
    	@Nullable
    	public String resolveStringValue(String strVal) throws BeansException { 
    		// 调用PropertyReplaceholderHelper替换字符串中的占位符，获取结果
    		String resolved = this.helper.replacePlaceholders(strVal, this.resolver);
    		if (trimValues) { 
    			resolved = resolved.trim();
    		}
    		return (resolved.equals(nullValue) ? null : resolved);
    	}
    }

[Link 1]: https://blog.csdn.net/wang0907/article/details/115483144