让我们从一个简单的问题开始:过滤器的层次结构。 有一个抽象类AbstractFilter和两个子类RegexFilter和StringMatchFilter 。
package bean.json.examples;public abstract class AbstractFilter {public abstract void filter();
}
这是RegexFilter类:
package bean.json.examples;public class RegexFilter extends AbstractFilter {private String pattern;public RegexFilter( final String pattern ) {this.pattern = pattern;}public void setPattern( final String pattern ) {this.pattern = pattern;}public String getPattern() {return pattern;}@Overridepublic void filter() {// Do some work here}
}
这是StringMatchFilter类:
package bean.json.examples;public class StringMatchFilter extends AbstractFilter {private String[] matches;private boolean caseInsensitive;public StringMatchFilter() {}public StringMatchFilter( final String[] matches, final boolean caseInsensitive ) {this.matches = matches;this.caseInsensitive = caseInsensitive;}public String[] getMatches() {return matches;}public void setCaseInsensitive( final boolean caseInsensitive ) {this.caseInsensitive = caseInsensitive;}public void setMatches( final String[] matches ) {this.matches = matches;}public boolean isCaseInsensitive() {return caseInsensitive;}@Overridepublic void filter() {// Do some work here}
}
没什么,纯Java Bean。 现在,如果我们需要将AbstractFilter实例的列表存储到JSON,更重要的是,要从JSON重新构造此列表,该怎么办? 以下类Filters演示了我的意思:
package bean.json.examples;import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;public class Filters {private Collection< AbstractFilter > filters = new ArrayList< AbstractFilter >();public Filters() {}public Filters( final AbstractFilter ... filters ) {this.filters.addAll( Arrays.asList( filters ) );}public Collection< AbstractFilter > getFilters() {return filters;}public void setFilters( final Collection< AbstractFilter > filters ) {this.filters = filters;}
}
由于JSON是文本的,平台无关的格式,因此它不包含任何类型特定的信息。 得益于出色的Jackson JSON处理器,它可以轻松完成。 因此,让我们将Jackson JSON处理器添加到我们的POM文件中:
<project><modelversion>4.0.0</modelversion><groupid>bean.json</groupid><artifactid>examples</artifactid><version>0.0.1-SNAPSHOT</version><packaging>jar</packaging><properties><project.build.sourceencoding>UTF-8</project.build.sourceencoding></properties><dependencies><dependency><groupid>org.codehaus.jackson</groupid><artifactid>jackson-mapper-asl</artifactid><version>1.9.6</version></dependency></dependencies></project>
完成此步骤后,我们需要告诉Jackson ,我们打算将类型信息与对象一起存储在JSON中,以便稍后可以从JSON重建确切的对象。 很少有AbstractFilter上的注释可以做到这一点。
import org.codehaus.jackson.annotate.JsonSubTypes;
import org.codehaus.jackson.annotate.JsonSubTypes.Type;
import org.codehaus.jackson.annotate.JsonTypeInfo;
import org.codehaus.jackson.annotate.JsonTypeInfo.Id;@JsonTypeInfo( use = Id.NAME )
@JsonSubTypes({@Type( name = "Regex", value = RegexFilter.class ),@Type( name = "StringMatch", value = StringMatchFilter.class )}
)
public abstract class AbstractFilter {// ...
}
而且...就是这样! 跟随帮助器类的工作是肮脏的工作,即将过滤器序列化为字符串,然后使用Jackson的 ObjectMapper从字符串反序列化它们:
package bean.json.examples;import java.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;import org.codehaus.jackson.map.ObjectMapper;public class FilterSerializer {private final ObjectMapper mapper = new ObjectMapper();public String serialize( final Filters filters ) {final StringWriter writer = new StringWriter();try {mapper.writeValue( writer, filters );return writer.toString();} catch( final IOException ex ) {throw new RuntimeException( ex.getMessage(), ex );} finally {try { writer.close(); } catch ( final IOException ex ) { /* Nothing to do here */ }}}public Filters deserialize( final String str ) {final StringReader reader = new StringReader( str );try {return mapper.readValue( reader, Filters.class );} catch( final IOException ex ) {throw new RuntimeException( ex.getMessage(), ex );} finally {reader.close();}}
}
让我们看看实际情况。 以下代码示例
final String json = new FilterSerializer().serialize(new Filters(new RegexFilter( "\\d+" ),new StringMatchFilter( new String[] { "String1", "String2" }, true ))
);
产生以下JSON:
{ "filters":[{"@type":"Regex","pattern":"\\d+"},{"@type":"StringMatch","matches":["String1","String2"],"caseInsensitive":true}]
}
如您所见, “ filters”集合中的每个条目都具有属性“ @type” ,该属性具有我们通过注释AbstractFilter类指定的值。 调用new FilterSerializer()。deserialize(json)会生成完全相同的Filters对象实例。
参考:我们的JCG合作伙伴 Andrey Redko在Andriy Redko {devmind}博客上提供了用于多态Java对象序列化的JSON 。
翻译自: https://www.javacodegeeks.com/2012/05/json-for-polymorphic-java-object.html