6. Singleton 本質：控制實例數目
6.1 EAGER SINGLETON
public class AppConfigEager {
private AppConfigEager() {
readConfig();
}
private static final AppConfigEager INSTANCE = new AppConfigEager();
public static AppConfigEager getInstance() {
return INSTANCE;
}
private String parameterA;
private String parameterB;
public String getParameterA() {
return parameterA;
}
public String getParameterB() {
return parameterB;
}
private void readConfig() {
Properties p = new Properties();
InputStream in = null;
try {
in = AppConfigEager.class.getResourceAsStream("AppConfig.properties");
p.load(in);
this.parameterA = p.getProperty("paramA");
this.parameterB = p.getProperty("paramB");
} catch (Exception e) {
System.out.println("Load app config file error.");
e.printStackTrace();
} finally {
try {
in.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
6.2 LAZY INITIALIZATION HOLDER SINGLETON
public class Singleton {
private Singleton(){
}
private static class SingletonLazyInitHolder {
private static Singleton instance = new Singleton();
}
public static Singleton getInstance() {
return SingletonLazyInitHolder.instance;
}
}
6.3 ENUM SINGLETON
public enum SingletonEnum {
INSTANCE;
private int count = 0;
public void countPlusOne() {
count++;
System.out.println("count: " + count);
}
}
public class SingletonEnumClient {
public static void main(String[] args) {
SingletonEnum.INSTANCE.countPlusOne();
SingletonEnum.INSTANCE.countPlusOne();
SingletonEnum.INSTANCE.countPlusOne();
}
}