常用单例写法

方法一：
public class PropertyConfig {
    private static class PropertyConfigInstance {
        private static final PropertyConfig _instance = new PropertyConfig();
    }
    public static PropertyConfig instance() {
        return PropertyConfigInstance._instance;
    }
        public String get(String key, String defaultValue) {
        String val = get(key);
        return val == null ? defaultValue : val;
    }
}
用法：
String  username = PropertyConfig.instance().get("username","test133").trim();

方法二：

public class HttpClientRequest {
    
    private static final HttpClientRequest INSTANCE_HTTP = new HttpClientRequest();
    public static HttpClientRequest instance() {
        return INSTANCE_HTTP;
    }

    public String sendMessageByHTTP(){
        ...
    } 
}    

用法：sendResult=HttpClientRequest.instance().sendMessageByHTTP();