Java NIO Path，javaniopath

Java NIO Path，javaniopath

The Java Path interface is part of the Java NIO 2 update which Java NIO received in Java 6 and Java 7. The Java Path interface was added to Java NIO in Java 7. The Path interface is located in the java.nio.file package, so the fully qualified name of the Java Path interface is java.nio.file.Path.

A Java Path instance represents a path in the file system. A path can point to either a file or a directory. A path can be absolute or relative. An absolute path contains the full path from the root of the file system down to the file or directory it points to. A relative path contains the path to the file or directory relative to some other path. Relative paths may sound a bit confusing. Don't worry. I will explain relative paths in more detail later in this Java NIO Path tutorial.

Do not confuse a file system path with the path environment variable in some operating systems. The java.nio.file.Path interface has nothing to do with the path environment variable.

In many ways the java.nio.file.Path interface is similar to the java.io.File class, but there are some minor differences. In many cases though, you can replace the use of the File class with use of the Path interface.

Creating a Path Instance

In order to use a java.nio.file.Path instance you must create a Path instance. You create a Path instance using a static method in the Paths class (java.nio.file.Paths) named Paths.get(). Here is a Java Paths.get() example:

import java.nio.file.Path;
import java.nio.file.Paths;

public class PathExample {

    public static void main(String[] args) {

        Path path = Paths.get("c:\\data\\myfile.txt");

    }
}

Notice the two import statements at the top of the example. To use the Path interface and the Paths class we must first import them.

Second, notice the Paths.get("c:\\data\\myfile.txt") method call. It is the call to the Paths.get() method that creates the Path instance. The Paths.get() method is a factory method for Path instances, in other words.

Creating an Absolute Path

Creating an absolute path is done by calling the Paths.get() factory method with the absolute file as parameter. Here is an example of creating a Path instance representing an absolute path:

Path path = Paths.get("c:\\data\\myfile.txt");

The absolute path is c:\data\myfile.txt. The double \ characters are necessary in Java strings, since the \ is an escape character, meaning the following character tells what character is really to be located at this place in the string. By writing \\ you tell the Java compiler to write a single \ character into the string.

The above path is a Windows file system path. On a Unix system (Linux, MacOS, FreeBSD etc.) the above absolute path could look like this:

Path path = Paths.get("/home/jakobjenkov/myfile.txt");

The absolute path is now /home/jakobjenkov/myfile.txt .

If you used this kind of path on a Windows machine (a path starting with /) the path would be interpreted as relative to the current drive. For instance, the path

/home/jakobjenkov/myfile.txt

could be interpreted as being located on the C drive. Then the path would correspond to this full path:

C:/home/jakobjenkov/myfile.txt

Creating a Relative Path

A relative path is a path that points from one path (the base path) to a directory or file. The full path (the absolute path) of a relative path is derived by combining the base path with the relative path.

The Java NIO Path class can also be used to work with relative paths. You create a relative path using the Paths.get(basePath, relativePath) method. Here are two relative path examples in Java:

Path projects = Paths.get("d:\\data", "projects");

Path file     = Paths.get("d:\\data", "projects\\a-project\\myfile.txt");

The first example creates a Java Path instance which points to the path (directory) d:\data\projects. The second example creates a Path instance which points to the path (file) d:\data\projects\a-project\myfile.txt .

When working with relative paths there are two special codes you can use inside the path string. These codes are:

• .
• ..

The . code means "current directory". For instance, if you create a relative path like this:

Path currentDir = Paths.get(".");
System.out.println(currentDir.toAbsolutePath());

Then the absolute path the Java Path instance corresponds to will be the directory in which the application executing the above code is executed.

If the . is used in the middle of a path string it just means the same directory as the path was pointing to at that point. Here is an Path example illustrating that:

Path currentDir = Paths.get("d:\\data\\projects\.\a-project");

This path will correspond to the path:

d:\data\projects\a-project

The .. code means "parent directory" or "one directory up". Here is a Path Java example illustrating that:

Path parentDir = Paths.get("..");

The Path instance created by this example would correspond to the parent directory of the directory from which the application running this code was started.

If you use the .. code in the middle of a path string it will correspond to changing one directory up at that point in the path string. For instance:

String path = "d:\\data\\projects\\a-project\\..\\another-project";
Path parentDir2 = Paths.get(path);

The Java Path instance created by this example will correspond to this absolute path:

d:\data\projects\another-project

The .. code after the a-project directory changes directory up the the parent directory projects and then the path references down into the another-project directory from there.

The . and .. codes also work in combination with the two-string Paths.get() method. Here are two Java Paths.get() examples showing simple examples of that:

Path path1 = Paths.get("d:\\data\\projects", ".\\a-project");

Path path2 = Paths.get("d:\\data\\projects\\a-project",
                       "..\\another-project");

There are more ways that the Java NIO Path class can be used to work with relative paths. You will learn more about that later in this tutorial.

Path.normalize()

The normalize() method of the Path interface can normalize a path. Normalizing means that it removes all the . and .. codes in the middle of the path string, and resolves what path the path string refers to. Here is a Java Path.normalize() example:

String originalPath =
        "d:\\data\\projects\\a-project\\..\\another-project";

Path path1 = Paths.get(originalPath);
System.out.println("path1 = " + path1);

Path path2 = path1.normalize();
System.out.println("path2 = " + path2);

This Path example first creates a path string with a .. code in the middle. Then the example creates a Path instance from this path string, and prints that Path instance out (actually it prints Path.toString()).

The example then calls normalize() on the created Path instance, which returns a new Path instance. This new, normalized Path instance is then also printed out.

Here is the output printed from the above example:

path1 = d:\data\projects\a-project\..\another-project
path2 = d:\data\projects\another-project

As you can see, the normalized path does not contain the a-project\.. part, as this is redundant. The removed part adds nothing to the final absolute path.

link from : http://tutorials.jenkov.com/java-nio/path.html