Java getClassLoader（）。getResourceAsStream（文件名）

Question

I have a Maven project. My entire code is in the folder {PROJECT_ROOT}/src/main/java. When I write the code:

InputStream input = VertxApp.class.getClassLoader().getResourceAsStream("file.txt");
String result = getStringFromInputStream(input);
System.out.println(result);

I see that the code looks for the file named "file.txt" in the folder: {PROJECT_ROOT}/src/main/resources.

My question is why. Why doesn't it look for files in {PROJECT_ROOT}/src/main/java or in ${PROJECT_ROOT} for example?

Answer 1

Most Maven projects follow the structure of

src/
  main/
    java/
    resources/

You're intended to keep your non-code resources separate from your code. The getResourceAsStream method assumes this standard is followed, so it looks for your resource starting from the resources/ directory.

See Why are project layout resources are kept separate from Java sources? for some more detailed thoughts.

Answer 2

When you build a maven project or run it from an IDE supporting maven, the java files from src/main/java are compiled to class files and put in PROJECT_ROOT/target/classes .

The files from src/main/resources are copied to PROJECT_ROOT/target/classes as well.

When you call VertxApp.class.getClassLoader().getResourceAsStream("file.txt") you get the classloader for the VertxApp class and this classloader will try to load the given resource from the path where the class was loaded from.

If your VertxApp class is in the top package, the the class file will be in target/classes and the resource will be searched there as well as you specify a relative path of "file.txt" , therefore you must put it in src/main/resources .

If the VertxApp class is for example in a package named mypackage then the class file would be in target/classes/mypackage and you should have the file in src/main/resources/mypackage so that it ends up in target/classes/mypackage as well.

If you have the VertxApp class in a package but want to keep the file in src/main/resources (without a subdirectory), you must reference it as an absolute path like this:

VertxApp.class.getClassLoader().getResourceAsStream("/file.txt")