Java带资源的try语句(try-with-resources)

java7中的新概念，try后面加括号，括号内可以写是使用的资源(创造对象)。
effective java建议，使用try-with-resources代替try-finally

比较能代表这个功能的是下列代码

public static void copy(String src, String dst) throws IOException {
    try (InputStream in = new FileInputStream(src);
         OutputStream out = new FileOutputStream(dst)) {
        byte[] buff = new byte[1024];
        int n;
        while ((n = in.read(buff)) >= 0) {
            out.write(buff, 0, n);
        }
    }
}

并不是所有的对象都可以在try后的括号内创建，该对象要实现AutoCloseable接口，如，我测试建立的类HHH

public class HHH implements AutoCloseable {
    @Override
    public void close() throws Exception {
        System.out.println("hhhh");
        throw new BusinessException("123");
    }

    public void hhh() {
        System.out.println("ttthhh");
        throw new BusinessException("567");
    }
}

其实只看这个，不是很理解到底是如何实现的，那就直接看反编译的class就行了(Idea就是好)

Test代码

public static void copy(String src, String dst) throws IOException {
    try (InputStream in = new FileInputStream(src);
         OutputStream out = new FileOutputStream(dst)) {
        byte[] buff = new byte[1024];
        int n;
        while ((n = in.read(buff)) >= 0) {
            out.write(buff, 0, n);
        }
    }
}

public static void testHHH() throws Exception {
    try (HHH h = new HHH()) {
        h.hhh();
    }
}

反编译后的代码

public static void copy(String src, String dst) throws IOException {
    InputStream in = new FileInputStream(src);
    Throwable var3 = null;

    try {
        OutputStream out = new FileOutputStream(dst);
        Throwable var5 = null;

        try {
            byte[] buff = new byte[1024];

            int n;
            while((n = in.read(buff)) >= 0) {
                out.write(buff, 0, n);
            }
        } catch (Throwable var29) {
            var5 = var29;
            throw var29;
        } finally {
            if (out != null) {
                if (var5 != null) {
                    try {
                        out.close();
                    } catch (Throwable var28) {
                        var5.addSuppressed(var28);
                    }
                } else {
                    out.close();
                }
            }

        }
    } catch (Throwable var31) {
        var3 = var31;
        throw var31;
    } finally {
        if (in != null) {
            if (var3 != null) {
                try {
                    in.close();
                } catch (Throwable var27) {
                    var3.addSuppressed(var27);
                }
            } else {
                in.close();
            }
        }

    }

}

public static void testHHH() throws Exception {
    HHH h = new HHH();
    Throwable var1 = null;

    try {
        h.hhh();
    } catch (Throwable var10) {
        var1 = var10;
        throw var10;
    } finally {
        if (h != null) {
            if (var1 != null) {
                try {
                    h.close();
                } catch (Throwable var9) {
                    var1.addSuppressed(var9);
                }
            } else {
                h.close();
            }
        }

    }

}

使用try-with-resources的目的，也是要保证资源的关闭(继承的AutoCloseable接口有close()抽象方法)，即使关闭失败，也要成功返回异常