Author: Thomas Lemmé

I am a Java Developer currently working at one of the top APM vendors as a Java agent developer.

Update slf4j Log Level Programmatically

Slf4j is an API that sits on top of a logging framework, e.g. Log4j 2, Logback or java.util.logging. So If you want to change the log level, the best way to do this is via the configuration of your logging implementation:

But sometimes, especially in tests it would be nice to see some logging output without changing global configuration files. Slf4j makes this very simple, just get the logger you want to change and set the log level:

Logger restClientLogger = (Logger) LoggerFactory.getLogger(SolrRestClient.class);
restClientLogger.setLevel(Level.DEBUG);

For the root logger you can use

Logger root = (Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
root.setLevel(Level.INFO);

 

 

Filter Solr for “Future Date or Not Set”

I’ve worked with some expression languages in my life. I have even created one for my masters thesis. Some of them are easy, some are complicated, some feel smooth, some are strange.

Recently Solr came to my life. Solr also supports some kind of expressions for queries and filters.

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The Java Class File Constant Pool

Everything that is constant in a class file is reflected in the constant pool. This means not only string or numeric constants, but everything that does not change during runtime, e.g.: variable and method names, method signatures, class names etc.

The information contained in the constant pool can be used to better understand the Java compiler or to do some static analysis.

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Java Class File Major & Minor Version

Every Java developer at some point comes across the following Exception and wonders what the strange numbers mean.

Exception in thread "main" java.lang.UnsupportedClassVersionError: java\SampleClass : Unsupported major.minor version 51.0

This happens when the class file has been compiled for a newer Java version than the version of the runtime, that is trying to load the class. But why the heck is it such a strange version number like 51.0 and what version of Java is this?

In the third part of the series about the Java Class File Format I take a look at the minor and major version information.

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0xCAFEBABE – The Java Class File Magic Number

As we saw in the last post each Java class file starts with the so-called “magic” section. This has historic reasons and goes back to the very early days of the Java language. You may take a look here or here for more details about James Goslings decision.

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The Java Class File Format – an Overview

The Java Class File Format is the format in which Java classes are stored when Java source files are compiled by the Java compiler. It includes all the information from the source files as well as some optimizations, but in a format the JVM can handle.

A complete description can be found in chapter 4 of the Java Virtual Machine Specification.

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Learning the Java Class File Format

Did you ever read that ("string 1" == "string 1") == true but ("string 1" == new String("string 1")) == false? Ever came across the terms “Constant Pool“, “StackMapTable” or the “java.lang.UnsupportedClassVersionError: Unsupported major.minor version” exception?

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Let’s gain some knowledge!

So I start this blogging thing. Again. But Why?

To gain some knowledge, learn and examine some stuff and write about it. So you, my dear reader, may benefit from my wisdom.

There are so many interesting things in technology out there. So many things to learn, to try, to read about. And to write about, because writing is one of the best forms of learning. As a side-effect, I hope my English writing will improve a little bit too.