以编程方式配置记录器

Question

我想以编程方式配置我的 java.util.logging.Logger。根据Oracle Docs，好像可以。

但是这个例子表明这里有问题：

public static void main(String[] args) {
    Logger logger = Logger.getLogger("org.acme.project");
    logger.setLevel(Level.SEVERE);
    logger = null;

    System.gc();

    logger = Logger.getLogger("org.acme.project");
    logger.warning("You shouldn't see this warning!");
    logger.severe("But this error!");
}

如果您运行代码，您将看到这两条消息。但是，如果您删除 logger = null; 或 System.gc();，您只会看到正确的第二个，这证明垃圾收集器只是删除了配置的记录器，而留下 Logger.getLogger(String) 并创建了一个新的（默认）记录器。

为了 "solve" 这个，我可以在某处保留对那个特定记录器的引用，但我认为这不是个好主意。

那么如何以编程方式定义记录器？

Answer 1

To "solve" this, I could just hold a reference to that particular logger somewhere, but I don't think that's a good idea.

在 JDK6u18 the LogManager held strong reference to the logger. After that patch, the java.util.logging.Logger.getLogger() 方法之前的版本中指向 reader 持有强引用：

Note: The LogManager may only retain a weak reference to the newly created Logger. It is important to understand that a previously created Logger with the given name may be garbage collected at any time if there is no strong reference to the Logger. In particular, this means that two back-to-back calls like getLogger("MyLogger").log(...) may use different Logger objects named "MyLogger" if there is no strong reference to the Logger named "MyLogger" elsewhere in the program.

一个常见的习语是使用：

private static final String CLASS_NAME = Foo.class.getName();
private static final Logger logger = Logger.getLogger(CLASS_NAME);

您也定义 CLASS_NAME 的原因是您会将其用于日志跟踪方法或 logp 方法

此外，像 FindBugs 这样的工具会检测到这种丢失的记录器模式：

LG: Potential lost logger changes due to weak reference in OpenJDK (LG_LOST_LOGGER_DUE_TO_WEAK_REFERENCE)

Answer 2

Logger#getLogger 的 Javadoc 引用了您的问题：

https://docs.oracle.com/javase/7/docs/api/java/util/logging/Logger.html#getLogger%28java.lang.String%29

Note: The LogManager may only retain a weak reference to the newly created Logger. It is important to understand that a previously created Logger with the given name may be garbage collected at any time if there is no strong reference to the Logger. In particular, this means that two back-to-back calls like getLogger("MyLogger").log(...) may use different Logger objects named "MyLogger" if there is no strong reference to the Logger named "MyLogger" elsewhere in the program.

反过来 Logger#getLogger 调用 LogManager#addLogger 并且其文档也鼓励您持有参考：

https://docs.oracle.com/javase/7/docs/api/java/util/logging/LogManager.html#addLogger%28java.util.logging.Logger%29

The application should retain its own reference to the Logger object to avoid it being garbage collected. The LogManager may only retain a weak reference.

以编程方式配置记录器

Configure Logger Programmatically

java

java.util.logging