缓存对象需要知道对象占用空间的大小,可以事先设置好每种类型的大小,此方法对普通的对象起效,Jive论坛中的对象也是采用这种办法来获取对象的大小的(取自Jive).
public class CacheSizes { /** * Returns the size in bytes of a basic Object. This method should only * be used for actual Object objects and not classes that extend Object. * * @return the size of an Object. */ public static int sizeOfObject() { return 4; } /** * Returns the size in bytes of a String. * * @param string the String to determine the size of. * @return the size of a String. */ public static int sizeOfString(String string) { if (string == null) { return 0; } return 4 + string.length()*2; } /** * Returns the size in bytes of a primitive int. * * @return the size of a primitive int. */ public static int sizeOfInt() { return 4; } /** * Returns the size in bytes of a primitive char. * * @return the size of a primitive char. */ public static int sizeOfChar() { return 2; } /** * Returns the size in bytes of a primitive boolean. * * @return the size of a primitive boolean. */ public static int sizeOfBoolean() { return 1; } /** * Returns the size in bytes of a primitive long. * * @return the size of a primitive long. */ public static int sizeOfLong() { return 8; } /** * Returns the size in bytes of a primitive double. * * @return the size of a primitive double. */ public static int sizeOfDouble() { return 8; } /** * Returns the size in bytes of a Date. * * @return the size of a Date. */ public static int sizeOfDate() { return 12; } /** * Returns the size in bytes of a Properties object. All properties and * property names must be Strings. * * @param properties the Properties object to determine the size of. * @return the size of a Properties object. */ public static int sizeOfProperties(Properties properties) { if (properties == null) { return 0; } //Base properties object int size = 36; //Add in size of each property Enumeration enum = properties.elements(); while(enum.hasMoreElements()) { String prop = (String)enum.nextElement(); size += sizeOfString(prop); } //Add in property names enum = properties.propertyNames(); while(enum.hasMoreElements()) { String prop = (String)enum.nextElement(); size += sizeOfString(prop); } return size; } /** * Returns the size in bytes of a Map object. All keys and * values <b>must be Strings</b>. * * @param map the Map object to determine the size of. * @return the size of the Map object. */ public static int sizeOfMap(Map map) { if (map == null) { return 0; } //Base map object -- should be something around this size. int size = 36; //Add in size of each value Iterator iter = map.values().iterator(); while(iter.hasNext()) { String value = (String)iter.next(); size += sizeOfString(value); } //Add in each key iter = map.keySet().iterator(); while(iter.hasNext()) { String key = (String)iter.next(); size += sizeOfString(key); } return size; } }
