I am looking for a Java equivalent for python's with statement, and I read about implementing the AutoCloseable interface and using try with resources.

In python, the context manager (with statement) uses two methods: __enter__ and __exit__, but in Java, the try with resources block uses only close, which is the equivalent of __exit__.

Is there an equivalent for the __enter__ method, in order to perform a certain method automatically when entering the try with resources block, and not only when the block is over?

CodePudding user response：

The equivalent is basically whatever you are calling in the try to get an instance of your AutoCloseable. This could be a constructor like:

try (MyClass obj = new MyClass()) {

Where the constructor is like:

public class MyClass implements AutoCloseable {
    public MyClass() {
        // do "enter" things...
    }
}

Depending on what you need "enter" to do, you might instead prefer a static producer for your class, which would look like this:

try (MyClass obj = MyClass.getInstance(someProperties)) {

Then your class might look something like this:

public class MyClass implements AutoCloseable {
    private MyClass() {
        // instantiate members
    }

    public static MyClass getInstance(Properties config) {
        // you could implement a singleton pattern or something instead, for example
        MyClass obj = new MyClass();
        // read properties...
        // do "enter" things...
        return obj;
    }
}

You could even call a factory or builder pattern in the try to produce your AutoCloseable. It all depends on your design and what you need the instance to do on "enter".

CodePudding user response：

In a Try with Resource like this:

try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {
    writer.println("Hello World");
}

Three actions occur:

• The resource writer is acquired in braces: (PrintWriter writer = new PrintWriter(new File("test.txt")))

• The resource is used inside curly braces: writer.println("Hello World");

• Resource is closed and freed up automatically when we exit the Try with Resource block

And this resource only is accessible inside corresponding block.