Java中的Try with Resources语句介绍

1.介绍

从Java7诞生了try-with-resources，这家伙可以在资源使用完后实现自动关闭回收。想想我们之前打开一个文件或流对象用完咋整的，是不是finally语句块中手动close的。

当然这类可自动关闭的资源前提是必须实现了AutoCloseable接口。

2.如何使用？

拿PrintWriter对象举例，资源必须在try结构中声明和初始化：

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

看下JDK1.8中PrintWriter的定义:

All Implemented Interfaces:

Closeable, Flushable, Appendable, AutoCloseable

3.使用try-with-resources替代try-catch-finally

Scanner scanner = null;
try {
    scanner = new Scanner(new File("test.txt"));
    while (scanner.hasNext()) {
        System.out.println(scanner.nextLine());
    }
} catch (FileNotFoundException e) {
    e.printStackTrace();
} finally {
    if (scanner != null) {
        scanner.close();
    }
}

• 下面是try-resources

try (Scanner scanner = new Scanner(new File("test.txt"))) {
    while (scanner.hasNext()) {
        System.out.println(scanner.nextLine());
    }
} catch (FileNotFoundException fnfe) {
    fnfe.printStackTrace();
}

4.同时使用多个资源

可以在一个try-with-resources块中用分号分隔以声明多个资源。例如：

try (Scanner scanner = new Scanner(new File("testRead.txt"));
    PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
    while (scanner.hasNext()) {
	writer.print(scanner.nextLine());
    }
}

5.自定义一个实现了AutoCloseable接口的资源

前提是要实现Closeable或AutoCloseable接口，同时重写close方法。例如：

public class MyResource implements AutoCloseable {
    @Override
    public void close() throws Exception {
        System.out.println("Closed MyResource");
    }
}

6.资源的关闭顺序是怎样的？

这里遵循的是：最先被定义/获取的资源最后被关闭。以下为例：

• 资源1

public class AutoCloseableResourcesFirst implements AutoCloseable {

    public AutoCloseableResourcesFirst() {
        System.out.println("Constructor -> AutoCloseableResources_First");
    }

    public void doSomething() {
        System.out.println("Something -> AutoCloseableResources_First");
    }

    @Override
    public void close() throws Exception {
        System.out.println("Closed AutoCloseableResources_First");
    }
}

• 资源2

public class AutoCloseableResourcesSecond implements AutoCloseable {

    public AutoCloseableResourcesSecond() {
        System.out.println("Constructor -> AutoCloseableResources_Second");
    }

    public void doSomething() {
        System.out.println("Something -> AutoCloseableResources_Second");
    }

    @Override
    public void close() throws Exception {
        System.out.println("Closed AutoCloseableResources_Second");
    }
}

• 测试

private void orderOfClosingResources() throws Exception {
    try (AutoCloseableResourcesFirst af = new AutoCloseableResourcesFirst();
        AutoCloseableResourcesSecond as = new AutoCloseableResourcesSecond()) {

        af.doSomething();
        as.doSomething();
    }
}

输出：

Constructor -> AutoCloseableResources_First
Constructor -> AutoCloseableResources_Second
Something -> AutoCloseableResources_First
Something -> AutoCloseableResources_Second
Closed AutoCloseableResources_Second
Closed AutoCloseableResources_First

7.Java9中的改进

来到java9我们可以在try-with-resources块中使用final或effectively final变量：

final Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))
try (scanner;writer) { 
    // omitted
}

如上，scanner变量声明为final，而writer变量没有显式声明final，但是很明显它在第一次赋值后并没有做其它改变，java8开始自动认为其为final变量。这一特性也就是effectively final。