public class SchoolServiceImpl {

	private static SchoolServiceImpl instance = new SchoolServiceImpl();

	private static ClassServiceImpl classServiceImpl = ClassServiceImpl.getInstanse();

	public static SchoolServiceImpl getInstanse() {

		if (instance == null)

			return instance = new SchoolServiceImpl();

		return instance;

	}

	public int getSchoolCount() {

		return classServiceImpl.getClassesCount();

	}

	public static void main(String[] args) {

		System.out.println(classServiceImpl.getClassesCount());

	}

}

public class ClassServiceImpl {

	/**

	 * @param args

	 */

	private static ClassServiceImpl instance = new ClassServiceImpl();

	public static  ClassServiceImpl getInstanse() {

	if (instance == null)

			return instance = new ClassServiceImpl();

		return instance;

	}

	private static SchoolServiceImpl schoolServiceImpl=SchoolServiceImpl.getInstanse();

	public int getClassesCount()

	{

		System.out.println(classServiceImpl.toString());

		return schoolServiceImpl.getSchoolCount();

	}

}

如上两个类彼此依赖当中的方法。导致死循环，终于导致内存溢出。

从中得到的教训：尽可能降低类的彼此依赖，做到单向依赖。