try-with-resources

try-with-resources 是Java7中增加的语法,相比于之前的try-catch-finally更简洁,更安全.

下面就让我们来看看他们之间的区别:

语法:

try-with-resources 支持自动按相反顺序关闭资源

try(多个资源){
    可能发生异常的业务代码;
}

try-catch-finally

try{
     可能发生异常的业务代码;
}catch(捕获的异常){
     
}fianlly{
     关闭资源
}

关闭io原来try-catch-finally:

1 static String readFirstLineFromFileWithFinallyBlock(String path)
2                                                     throws IOException {
3     BufferedReader br = new BufferedReader(new FileReader(path));
4     try {
5         return br.readLine();
6     } finally {
7         if (br != null) br.close();
8     }
9 }

其实这样在finally中还是可能会发生异常,所以方法还是要抛出异常(以防万一)

static String readFirstLineFromFile(String path) throws IOException {
    try (BufferedReader br =
                  new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

再看一组对比:

public static void writeToFileZipFileContents(String zipFileName,
                                          String outputFileName)
                                          throws java.io.IOException {

    java.nio.charset.Charset charset =
        java.nio.charset.StandardCharsets.US_ASCII;
    java.nio.file.Path outputFilePath =
        java.nio.file.Paths.get(outputFileName);

    // Open zip file and create output file with
    // try-with-resources statement

    try (
        java.util.zip.ZipFile zf =
            new java.util.zip.ZipFile(zipFileName);
        java.io.BufferedWriter writer =
            java.nio.file.Files.newBufferedWriter(outputFilePath, charset)
    ) {
        // Enumerate each entry
        for (java.util.Enumeration entries =
                                zf.entries(); entries.hasMoreElements();) {
            // Get the entry name and write it to the output file
            String newLine = System.getProperty("line.separator");
            String zipEntryName =
                ((java.util.zip.ZipEntry)entries.nextElement()).getName() +
                newLine;
            writer.write(zipEntryName, 0, zipEntryName.length());
        }
    }
}

public static void viewTable(Connection con) throws SQLException {

    String query = "select COF_NAME, SUP_ID, PRICE, SALES, TOTAL from COFFEES";

    try (Statement stmt = con.createStatement()) {
        ResultSet rs = stmt.executeQuery(query);

        while (rs.next()) {
            String coffeeName = rs.getString("COF_NAME");
            int supplierID = rs.getInt("SUP_ID");
            float price = rs.getFloat("PRICE");
            int sales = rs.getInt("SALES");
            int total = rs.getInt("TOTAL");

            System.out.println(coffeeName + ", " + supplierID + ", " +
                              price + ", " + sales + ", " + total);
        }
    } catch (SQLException e) {
        JDBCTutorialUtilities.printSQLException(e);
    }
}

注意：try-with-resources语句也可以像普通的try语句一样，有catch和finally代码块。在try-with-resources语句中，任何的catch和finally代码块都在所有被声明的资源被关闭后执行。

被压抑的异常

try-with-resources语句相关联的代码块可能会抛出异常。在writeToFileZipFileContents例子中，try代码块中可能会抛出异常，并且有高达两个异常可能会在try-with-resources语句抛出，当试图去关闭ZipFile和BufferedWriter对象的时候。如果在try代码块中抛出一个异常，同时在try-with-resources语句中抛出一个或多个异常，那么在try-with-resources语句中抛出的异常就被压抑了，并且最终在writeToFileZipFileContents方法抛出的异常就是try代码块中抛出的那个异常。你可以通过被try代码块抛出的异常的Throwable.getSuppressed方法找回被压抑的异常。

实现了AutoCloseable或Closeable接口的类

看 AutoCloseable 和 Closeable 接口的javadoc，看看实现了它们的类都有哪些。Closeable接口继承了AutoCloseable接口。不同之处在于，Closeable接口的close方法抛出的是IOException异常，AutoCloseable接口的close方法抛出的是Exception异常。因此，AutoCloseable的子类可以重写close方法的行为，抛出指定的异常，比如IOException异常，甚至可以不抛出异常。

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