用枚举实现工厂模式

public enum AgentType {
	HTTP("http") {
		public Agent newInstance() {
			return new HttpAgent();
		}
	},
	WEBSERVICE("webSerivce") {
		public Agent newInstance() {
			return new WSAgent();
		}
	};

	public abstract Agent newInstance();

	private String op;

	private AgentType(String op) {
		this.op = op;
	}

	public String getOp() {
		return op;
	}

	public static Agent getAgent(String op) {
		for (AgentType type : AgentType.values()) {
			if (type.getOp() == op) {
				return type.newInstance();
			}
		}
		return null;
	}
}