2020年4月8日 00:21:37go评论69阅读模式

英文:

Creating a parser of Class name + String value to a typed value

问题

我正在尝试编写一个方法，该方法可以接受一个字符串类名和一个字符串值，并将该字符串表示的值返回。

示例输入：

parse("java.lang.String", "abc") -> String "ABC"
parse("java.lang.Boolean", "FALSE") -> Boolean FALSE
parse("java.lang.Integer", "123") -> Integer 123
parse("com.me.Color", "RED") -> enum Color.RED


我发现，如果我使用一个包含`assignableFrom`调用的if块，我可以实现这一点。但我更希望编写一些更具扩展性的代码，这样明天就不会那么难添加一个新的解析器。

这是我现在的代码：

```java
    String stringClassName = //stringified full class name
    String value = //value to parse
    Class&lt;?&gt; fieldType = Class.forName(stringClassName)
    if (fieldType.isAssignableFrom(String.class)) {
      return value;
    } else if (fieldType.isAssignableFrom(Boolean.class)) {
      return Util.toBoolean(value);
    } else if (fieldType.isEnum()) {
      return Util.toEnum(fieldType, value);
    } else {
      // throw exception
    }


<details>
<summary>英文:</summary>

I am trying to write a method that can take in a String classname and a String value, and return the value represented as that String. 

Example inputs:

parse("java.lang.String", "abc") -> String "ABC"
parse("java.lang.Boolean", "FALSE") -> Boolean FALSE
parse("java.lang.Integer", "123") -> Integer 123
parse("com.me.Color", "RED") -> enum Color.RED


I have found that if I use an if block containing `assignableFrom` calls, I can achieve this. But would prefer writing something more extendable, so it isn&#39;t as difficult to add a new parser tomorrow. 

This is what I have now:

String stringClassName = //stringified full class name
String value = //value to parse
Class&lt;?&gt; fieldType = Class.forName(stringClassName)
if (fieldType.isAssignableFrom(String.class)) {
  return value;
} else if (fieldType.isAssignableFrom(Boolean.class)) {
  return Util.toBoolean(value);
} else if (fieldType.isEnum()) {
  return Util.toEnum(fieldType, value);
} else {
  // throw exception
}


</details>


# 答案1
**得分**: 2

以下是翻译好的代码部分：

```java
有多种方法可以做到这一点。例如：

您可以创建一个名为`Parser`的接口
```java
package example;

public interface Parser {

	boolean canParse(String fullQualifiedClassName);
	Object parse(String fullQualifiedClassName, String value) throws ParseException;

	class ParseException extends Exception {

		public ParseException(String msg) {
			super(msg);
		}

		public ParseException(Exception cause) {
			super(cause);
		}
	}
}

并且将所有默认实现放在一个枚举或以其他方式静态定义的地方：

package example;

public enum DefaultParser implements Parser {

	STRING {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, String.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			return value;
		}
	},
	ENUM {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, Enum.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			final Class<?> clazz;
			try {
				clazz = (Class<? extends Enum>) Class.forName(fullQualifiedClassName);
			} catch (ClassNotFoundException e) {
				throw new ParseException(e);
			}

			return Enum.valueOf(clazz, value);
		}
	},
	BOOLEAN {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, Boolean.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			return value.toLowerCase().equals("true");
		}
	};

	private static boolean isClassAssignableFromClassName(String fullQualifiedClassName, Class<?> clazz) {
		try {
			return clazz.isAssignableFrom(Class.forName(fullQualifiedClassName));
		} catch (ClassNotFoundException e) {
			return false;
		}
	}
}

还有一个ParentParser实现，将多个解析器组合成一个：

package example;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class ParentParser implements Parser {

	private final List<Parser> parsers;

	public ParentParser() {
		this.parsers = new ArrayList<>();
		this.parsers.addAll(Arrays.asList(DefaultParser.values()));
	}

	public void register(Parser parser) {
		this.parsers.add(parser);
	}

	@Override
	public boolean canParse(String fullQualifiedClassName) {
		return findParser(fullQualifiedClassName).isPresent();
	}

	@Override
	public Object parse(String fullQualifiedClassName, String value) throws ParseException {
		return findParser(fullQualifiedClassName)
			  .orElseThrow(() -> new ParseException("no registered parser found for class=" + fullQualifiedClassName))
			  .parse(fullQualifiedClassName, value);
	}

	private Optional<Parser> findParser(String fullQualifiedClassName) {
		return this.parsers.stream().filter(parser -> parser.canParse(fullQualifiedClassName)).findAny();
	}
}

然后可以像这样使用它：

package example;

import example.Parser.ParseException;

public class Example {

	public static void main(String[] args) throws ParseException {
		final ParentParser parser = new ParentParser();

		System.out.println(parser.parse("java.lang.String", "hello world"));
		System.out.println(parser.parse("java.lang.Boolean", "true"));
		System.out.println(parser.parse("java.time.DayOfWeek", "TUESDAY"));
	}
}

您还可以添加更多的解析器，例如使用Jackson（JSON）的解析器：

package example;

import com.fasterxml.jackson.databind.ObjectMapper;
import example.Parser.ParseException;

import java.io.IOException;

public class Example {

	public static void main(String[] args) throws ParseException {
		final ParentParser parser = new ParentParser();

		System.out.println(parser.parse("java.lang.String", "hello world"));
		System.out.println(parser.parse("java.lang.Boolean", "true"));
		System.out.println(parser.parse("java.time.DayOfWeek", "TUESDAY"));

		parser.register(new JacksonParser());

		System.out.println(parser.parse("java.util.Map", "{\"key\":\"value\"}"));
	}

	private static class JacksonParser implements Parser {

		private static final ObjectMapper MAPPER = new ObjectMapper();

		@Override
		public boolean canParse(String fullQualifiedClassName) {
			final Class<?> clazz;
			try {
				clazz = Class.forName(fullQualifiedClassName);
			} catch (ClassNotFoundException e) {
				return false;
			}

			return MAPPER.canDeserialize(MAPPER.constructType(clazz));
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			try {
				return MAPPER.readValue(value, Class.forName(fullQualifiedClassName));
			} catch (ClassNotFoundException | IOException e) {
				throw new ParseException(e);
			}
		}
	}
}

请注意，根据您的需求，可以进行优化。如果您的解析器实现只能解析静态类型列表，并且每个类只有一个解析器实现，您应该将List<Parser>更改为Map<Class<?>, Parser>，并将register方法更改为register(Class<?>, Parser)，例如。

英文:

There are multiple ways to do this. For example:

You could have an interface called Parser

package example;

public interface Parser {

	boolean canParse(String fullQualifiedClassName);
	Object parse(String fullQualifiedClassName, String value) throws ParseException;

	class ParseException extends Exception {

		public ParseException(String msg) {
			super(msg);
		}

		public ParseException(Exception cause) {
			super(cause);
		}
	}
}

And all your Default-Implementations in an Enum or statically defined in another way:

package example;

public enum DefaultParser implements Parser {

	STRING {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, String.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			return value;
		}
	},
	ENUM {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, Enum.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			final Class&lt;? extends Enum&gt; clazz;
			try {
				clazz = (Class&lt;? extends Enum&gt;) Class.forName(fullQualifiedClassName);
			} catch (ClassNotFoundException e) {
				throw new ParseException(e);
			}

			return Enum.valueOf(clazz, value);
		}
	},
	BOOLEAN {
		@Override
		public boolean canParse(String fullQualifiedClassName) {
			return isClassAssignableFromClassName(fullQualifiedClassName, Boolean.class);
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			return value.toLowerCase().equals(&quot;true&quot;);
		}
	};

	private static boolean isClassAssignableFromClassName(String fullQualifiedClassName, Class&lt;?&gt; clazz) {
		try {
			return clazz.isAssignableFrom(Class.forName(fullQualifiedClassName));
		} catch (ClassNotFoundException e) {
			return false;
		}
	}
}

And a ParentParser Implementation that combines multiple Parsers into one:

package example;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class ParentParser implements Parser {

	private final List&lt;Parser&gt; parsers;

	public ParentParser() {
		this.parsers = new ArrayList&lt;&gt;();
		this.parsers.addAll(Arrays.asList(DefaultParser.values()));
	}

	public void register(Parser parser) {
		this.parsers.add(parser);
	}

	@Override
	public boolean canParse(String fullQualifiedClassName) {
		return findParser(fullQualifiedClassName).isPresent();
	}

	@Override
	public Object parse(String fullQualifiedClassName, String value) throws ParseException {
		return findParser(fullQualifiedClassName)
			  .orElseThrow(() -&gt; new ParseException(&quot;no registered parser found for class=&quot; + fullQualifiedClassName))
			  .parse(fullQualifiedClassName, value);
	}

	private Optional&lt;Parser&gt; findParser(String fullQualifiedClassName) {
		return this.parsers.stream().filter(parser -&gt; parser.canParse(fullQualifiedClassName)).findAny();
	}
}

Which you can then use like this:

package example;

import example.Parser.ParseException;

public class Example {

	public static void main(String[] args) throws ParseException {
		final ParentParser parser = new ParentParser();

		System.out.println(parser.parse(&quot;java.lang.String&quot;, &quot;hello world&quot;));
		System.out.println(parser.parse(&quot;java.lang.Boolean&quot;, &quot;true&quot;));
		System.out.println(parser.parse(&quot;java.time.DayOfWeek&quot;, &quot;TUESDAY&quot;));
	}
}

And you could add more parsers, for example a parser using Jackson (JSON):

package example;

import com.fasterxml.jackson.databind.ObjectMapper;
import example.Parser.ParseException;

import java.io.IOException;

public class Example {

	public static void main(String[] args) throws ParseException {
		final ParentParser parser = new ParentParser();

		System.out.println(parser.parse(&quot;java.lang.String&quot;, &quot;hello world&quot;));
		System.out.println(parser.parse(&quot;java.lang.Boolean&quot;, &quot;true&quot;));
		System.out.println(parser.parse(&quot;java.time.DayOfWeek&quot;, &quot;TUESDAY&quot;));

		parser.register(new JacksonParser());

		System.out.println(parser.parse(&quot;java.util.Map&quot;, &quot;{\&quot;key\&quot;:\&quot;value\&quot;}&quot;));
	}

	private static class JacksonParser implements Parser {

		private static final ObjectMapper MAPPER = new ObjectMapper();

		@Override
		public boolean canParse(String fullQualifiedClassName) {
			final Class&lt;?&gt; clazz;
			try {
				clazz = Class.forName(fullQualifiedClassName);
			} catch (ClassNotFoundException e) {
				return false;
			}

			return MAPPER.canDeserialize(MAPPER.constructType(clazz));
		}

		@Override
		public Object parse(String fullQualifiedClassName, String value) throws ParseException {
			try {
				return MAPPER.readValue(value, Class.forName(fullQualifiedClassName));
			} catch (ClassNotFoundException | IOException e) {
				throw new ParseException(e);
			}
		}
	}
}

Note that this can of course be optimized depending on your needs.
If your Parser-Implementations can only parse a static List of Types and there is only one Parser-Implementation per Class, you should change the List<Parser> to Map<Class<?>, Parser> and change the register-Method to register(Class<?> clazz, Parser parser) for example

答案2

得分: 0

以下是翻译好的内容：

你可以使用 Java 中的反射 API 编写一个通用解决方案。
这将减少大量的代码量并且更具可扩展性。
同时，枚举类型需要单独的处理。
我在下面的代码中涵盖了基本情况。

public static void main(String[] args) throws NoSuchMethodException, ClassNotFoundException, IllegalAccessException, InvocationTargetException, InstantiationException {
    Object instance1 = parse("java.lang.String", "abc", false);
    Object instance2 = parse("java.lang.Boolean", "FALSE", false);
    Object instance3 = parse("java.lang.Integer", "123", false);
    Object instance4 = parse("com.me.Color", "RED", true);
}

private static Object parse(String className, String argument, boolean isEnum) throws NoSuchMethodException, ClassNotFoundException, IllegalAccessException, InstantiationException, InvocationTargetException {
    if (isEnum) {
        Object value = Enum.valueOf((Class<? extends Enum>) Class.forName(className), argument);
        //System.out.println(value);
        return value;
    } else {
        return parse(className, new Object[]{argument}, isEnum);
    }
}

private static Object parse(String className, Object[] arguments, boolean isEnum) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
    Class<?> clazz = Class.forName(className);
    Constructor<?> ctor = clazz.getConstructor(String.class);
    Object object = ctor.newInstance(arguments);
    //System.out.println(object);
    return object;
}

英文:

You can write a generic solution using reflection apis in java.
That would reduce a lot amount of code and would be more extensible.
Also not there is a separate processing required for enum types.
I have covered the basic cases in the code shown below.

public static void main(String[] args) throws NoSuchMethodException, ClassNotFoundException, IllegalAccessException, InvocationTargetException, InstantiationException {
Object instance1 = parse(&quot;java.lang.String&quot;, &quot;abc&quot;, false);
Object instance2 = parse(&quot;java.lang.Boolean&quot;, &quot;FALSE&quot;, false);
Object instance3 = parse(&quot;java.lang.Integer&quot;, &quot;123&quot;, false);
Object instance4 = parse(&quot;com.me.Color&quot;, &quot;RED&quot;, true);
}
private static Object parse(String className, String argument, boolean isEnum) throws NoSuchMethodException, ClassNotFoundException, IllegalAccessException, InstantiationException, InvocationTargetException {
if (isEnum) {
Object value = Enum.valueOf((Class&lt;? extends Enum&gt;) Class.forName(className), argument);
//System.out.println(value);
return value;
} else {
return parse(className, new Object[]{argument}, isEnum);
}
}
private static Object parse(String className, Object[] arguments, boolean isEnum) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
Class&lt;?&gt; clazz = Class.forName(className);
Constructor&lt;?&gt; ctor = clazz.getConstructor(String.class);
Object object = ctor.newInstance(arguments);
//System.out.println(object);
return object;
}

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