Slf4j 简明教程
SLF4J - Parameterized logging
正如本教程前面讨论的，SLF4J 提供对参数化日志消息的支持。
As discussed earlier in this tutorial SLF4J provides support for parameterized log messages.
您可以在消息中使用参数，并在同一语句的后面为它们传递值。
You can use parameters in the messages and pass values to them later in the same statement.
Syntax
如下所示，您需要在消息（字符串）中使用占位符（{}），只要您需要在 object 形式中传递值，并使用逗号分隔消息和值。
As shown below, you need to use placeholders ({}) in the message (String) wherever you need and later you can pass value for place holder in object form, separating the message and value with comma.
Integer age;
Logger.info("At the age of {} ramu got his first job", age);Example
以下示例演示使用 SLF4J 的参数化日志记录（带单个参数）。
The following example demonstrates parameterized logging (with single parameter) using SLF4J.
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class PlaceHolders {
public static void main(String[] args) {
//Creating the Logger object
Logger logger = LoggerFactory.getLogger(PlaceHolders.class);
Integer age = 23;
//Logging the information
logger.info("At the age of {} ramu got his first job", age);
}
}Output
执行后，上述程序生成以下输出 -
Upon execution, the above program generates the following output −
Dec 10, 2018 3:25:45 PM PlaceHolders main
INFO: At the age of 23 Ramu got his first jobAdvantage of Parameterized Logging
在 Java 中，如果我们需要在语句中打印值，我们将使用连接运算符，如下所示：
In Java, if we need to print values in a statement, we will use concatenation operator as −
System.out.println("At the age of "+23+" ramu got his first job");这涉及将整数值 23 转换为字符串，并将该值连接到它周围的字符串。
This involves the conversion of the integer value 23 to string and concatenation of this value to the strings surrounding it.
如果这是一个日志语句，并且该语句的特定日志级别被禁用，则所有这些计算都将毫无用处。
And if it is a logging statement, and if that particular log level of your statement is disabled then, all this calculation will be of no use.
在这种情况，您可以使用参数化日志记录。在该格式中，SLF4J 最初确认是否启用了特定级别的日志记录。如果是，则它用各自的值替换消息中的占位符。
In such circumstances, you can use parameterized logging. In this format, initially SLF4J confirms whether the logging for particular level is enabled. If so then, it replaces the placeholders in the messages with the respective values.
例如，如果我们有一个语句，如
For example, if we have a statement as
Integer age;
Logger.debug("At the age of {} ramu got his first job", age);只有在启用调试时，SLF4J 才会将年龄转换成整数，并在字符串连接它，否则它将不执行任何操作。因此，如果禁用日志记录级别，将导致参数构造成本。
Only if debugging is enabled then, SLF4J converts the age into integer and concatenates it with the strings otherwise, it does nothing. Thus incurring the cost of parameter constructions when logging level is disabled.
Two Argument Variant
您还可以在消息中使用两个参数，如下所示：
You can also use two parameters in a message as −
logger.info("Old weight is {}. new weight is {}.", oldWeight, newWeight);Example
以下示例演示了带参数日志记录中两个占位符的用法。
The following example demonstrates the usage of two placeholders in parametrized logging.
import java.util.Scanner;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class PlaceHolders {
public static void main(String[] args) {
Integer oldWeight;
Integer newWeight;
Scanner sc = new Scanner(System.in);
System.out.println("Enter old weight:");
oldWeight = sc.nextInt();
System.out.println("Enter new weight:");
newWeight = sc.nextInt();
//Creating the Logger object
Logger logger = LoggerFactory.getLogger(Sample.class);
//Logging the information
logger.info("Old weight is {}. new weight is {}.", oldWeight, newWeight);
//Logging the information
logger.info("After the program weight reduced is: "+(oldWeight-newWeight));
}
}Output
执行后，上述程序生成以下输出。
Upon execution, the above program generates the following output.
Enter old weight:
85
Enter new weight:
74
Dec 10, 2018 4:12:31 PM PlaceHolders main
INFO: Old weight is 85. new weight is 74.
Dec 10, 2018 4:12:31 PM PlaceHolders main
INFO: After the program weight reduced is: 11Multiple Argument Variant
您还可以使用多个占位符，如下面的示例所示：
You can also use more than two placeholders as shown in the following example −
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class PlaceHolders {
public static void main(String[] args) {
Integer age = 24;
String designation = "Software Engineer";
String company = "Infosys";
//Creating the Logger object
Logger logger = LoggerFactory.getLogger(Sample.class);
//Logging the information
logger.info("At the age of {} ramu got his first job as a {} at {}", age, designation, company);
}
}