Java Lambda表达式和日志记录

Question

我正在阅读Log4j2的新功能,并且有一个功能可以启用

"Java 8 lambda支持懒惰日志记录"

它给出了两个例子

第一个是不好的做法

// pre-Java 8 style optimization: explicitly check the log level
// to make sure the expensiveOperation() method is only called if necessary
 if (logger.isTraceEnabled()) {
  logger.trace("Some long-running operation returned {}", expensiveOperation());
 }

第二个是良好的做法

// Java-8 style optimization: no need to explicitly check the log level:
// the lambda expression is not evaluated if the TRACE level is not enabled
logger.trace("Some long-running operation returned {}", () -> expensiveOperation());

在哪里检查是否启用了请求的日志级别？ "logger.isTraceEnabled()"？

Answer 1

在哪里检查是否启用了请求的日志级别？

在logger.trace()方法里面.

这里的诀窍在于你传递参数的方式.pre-java8样式计算调用时的值logger.trace.

logger.trace(..., expensiveOperation());

Java 8风格使用了 Supplier

logger.trace( ..., () -> expensiveOperation());

所以expensiveOperation()只有在请求时才调用它 - 在trace方法内部.

看看实施情况java.util.logging.Logger.log():

public void log(Level level, Supplier<String> msgSupplier) {
    if (!isLoggable(level)) {
        return;
    }
    LogRecord lr = new LogRecord(level, msgSupplier.get()); // <-- here is the expensive computation
    doLog(lr);
}