单例模式：一个类只能创建一个实例。
类图:
实现方式有以下几种：
1、懒汉式：
public class Export {
private static Export export;
private Export() {
}
public static Export getInstance() {
if (export == null) {
export = new Export();
}
return export;
}
public void print(){
System.out.println("this is singleton");
}
}2、线程安全懒汉式：全部实例加锁
public class SyncFuncExport {
private static SyncFuncExport export;
private SyncFuncExport() {
}
public static synchronized SyncFuncExport getInstance() {
if (export == null) {
export = new SyncFuncExport();
}
return export;
}
public void print(){
System.out.println("this is synchronized function singleton");
}
}3、线程安全锁懒汉式：如果已初始化则没有锁，提高性能
public class SyncClassExport {
private static SyncClassExport export;
private SyncClassExport() {
}
public static SyncClassExport getInstance() {
if (export == null) {
synchronized (SyncClassExport.class) {
if (export == null) {
export = new SyncClassExport();
}
}
}
return export;
}
public void print(){
System.out.println("this is synchronized class singleton");
}
}
4、类加载时实例化，实现简单。
public class InitExport {
private static InitExport export = new InitExport();
private InitExport() {
}
public static InitExport getInstance() {
return export;
}
public void print(){
System.out.println("this is init singleton");
}
}
5、通过内部类，在使用时实例化
public class InnerClassExport {
private static class InnerClassExportInstance{
public static final InnerClassExport export = new InnerClassExport();
}
private InnerClassExport() {
}
public static InnerClassExport getInstance() {
return InnerClassExportInstance.export;
}
public void print(){
System.out.println("this is inner class singleton");
}
}6、为避免子类实例化，添加final，不能被继承
public final class InitExport {
private static InitExport export = new InitExport();
private InitExport() {
}
public static InitExport getInstance() {
return export;
}
public void print(){
System.out.println("this is init singleton");
}
}
