單例模式的幾種實現方式

/**考察知識點單例及Spring Bean的單例設計*/
class Singleton01{
  private Singleton01() {}
  private static Singleton01 instance;
  public static Singleton01 getInstance() {
 if(instance==null) {
 instance=new Singleton01();
 }
 return instance;
  }
}
class Singleton02{//適合大對象，稀少用
 private Singleton02() {}
 private static Singleton02 instance;
 public synchronized static Singleton02 getInstance() {
 if(instance==null) {
 instance=new Singleton02();
 }
 return instance;
 }
}
class Singleton03{//適合大對象(合適需要合適加載)，稀少用
 private Singleton03() {}
 private static volatile Singleton03 instance;
 public static Singleton03 getInstance() {
 if(instance==null) {
 synchronized (Singleton03.class) {
 if(instance==null) {
   instance=new Singleton03();
 }
 } 
 }
 return instance;
 }
}
class Singleton04{//小對象，頻繁用
//int[] n=new int[1024];
private Singleton04() {}
private static  Singleton04 instance=new Singleton04();
public static Singleton04 getInstance() {
return instance;
} 
public static void display() {}
}
class Singleton05{//對04進行優化
private Singleton05() {}
static class Inner{//延遲加載策略
private static  Singleton05 instance=
new Singleton05();
}
public static Singleton05  getInstance() {
return Inner.instance;
} 
public static void display() {}
public void show() {}
}
enum Singleton06{
instance;

public static void display() {}
}