Android中的注解

package com.cooliris.picasa;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;import org.xml.sax.Attributes;public abstract class Entry {    public static final String[] ID_PROJECTION = { "_id" };    // The primary key of the entry. by_zh    @Column("_id") //使用    public long id = 0;    @Retention(RetentionPolicy.RUNTIME) //使用失效。 注解存在于类文件中，而且在运行时虚拟机可以获取注解信息。 by_zh    @Target(ElementType.TYPE) //此注解只能用来对类、接口以及枚举类型进行注解 by_zh    public @interface Table {        String value();    }    @Retention(RetentionPolicy.RUNTIME)    @Target(ElementType.FIELD) //此注解只能用来对成员变量进行注解    public @interface Column {        String value(); //注解属性，value是获取此属性值的方法的名称。        boolean indexed() default false;        boolean fullText() default false;    }    public void clear() {        id = 0;    }    public void setPropertyFromXml(String uri, String localName, Attributes attrs, String content) {        throw new UnsupportedOperationException("Entry class does not support XML parsing");    }}

private static final class PhotoProjection extends Entry {        public static final EntrySchema SCHEMA = new EntrySchema(PhotoProjection.class);        @Column("edit_uri")        public String editUri;        @Column("title")        public String title;        @Column("summary")        public String summary;        @Column("date_taken")        public long dateTaken;        @Column("latitude")        public double latitude;        @Column("longitude")        public double longitude;        @Column("thumbnail_url")        public String thumbnailUrl;        @Column("screennail_url")        public String screennailUrl;        @Column("content_url")        public String contentUrl;        @Column("content_type")        public String contentType;        @Column("html_page_url")        public String htmlPageUrl;    }

下面是在另一个类中使用注解：

private ColumnInfo[] parseColumnInfo(Class<? extends Object> clazz) {        // Gather metadata from each annotated field.        ArrayList<ColumnInfo> columns = new ArrayList<ColumnInfo>();        Field[] fields = clazz.getFields();        for (int i = 0; i != fields.length; ++i) {            // Get column metadata from the annotation.            Field field = fields[i];            Entry.Column info = ((AnnotatedElement) field).getAnnotation(Entry.Column.class);            if (info == null) {                continue;            }            // Determine the field type.            int type;            Class<?> fieldType = field.getType();            if (fieldType == String.class) {                type = TYPE_STRING;            } else if (fieldType == boolean.class) {                type = TYPE_BOOLEAN;            } else if (fieldType == short.class) {                type = TYPE_SHORT;            } else if (fieldType == int.class) {                type = TYPE_INT;            } else if (fieldType == long.class) {                type = TYPE_LONG;            } else if (fieldType == float.class) {                type = TYPE_FLOAT;            } else if (fieldType == double.class) {                type = TYPE_DOUBLE;            } else if (fieldType == byte[].class) {                type = TYPE_BLOB;            } else {                throw new IllegalArgumentException("Unsupported field type for column: " + fieldType.getName());            }            // Add the column to the array.            int index = columns.size();            columns.add(new ColumnInfo(info.value(), type, info.indexed(), info.fullText(), field, index)); //使用注解 by_zh        }

@Entry.Table("photos")public final class PhotoEntry extends Entry {    public static final EntrySchema SCHEMA = new EntrySchema(PhotoEntry.class);    /**     * The user account that is the sync source for this entry. Must be set     * before insert/update.     */    @Column("sync_account")    public String syncAccount;    /**     * The "edit" URI of the photo.     */    @Column("edit_uri")    public String editUri;......}

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