Java NIO try-with-resources [reprint]

Java NIO Try -with-resources [reprint]

@ Original link http://blog.csdn.net/jackiehff/article/details/17765909

    tryThe-with-resources statement is a statement that declares one or more resources try . A resource, as an object, must be closed after the program ends. The try -with-resources statement ensures that every resource at the end of the statement is closed. Any implementation java.lang.AutoCloseable的对象 , including all implementations java.io.Closeable 的对象 , can be used as a resource.

The following example reads the first line of the file. It uses BufferedReader an instance to read the data in the file. BufferedReaderis a resource that must be closed after the program ends:

 1  static  String Readfirstlinefromfile (String path) throws   2  try  ( BufferedReader br = new  BufferedReader (new   FileReader (path)) { 3  return   Br.readline ();  4   5      }

In this example, the try -with-resources statement declares the resource to be one BufferedReader . The declaration statement is immediately try inside the parentheses of the keyword. In Java SE 7 and later BufferedReader类实现了 java.lang.AutoCloseable接口 . Because the BufferedReader instance is try declared in the-with-resource statement, either the try statement is completed normally or an accident occurs (the result is that the Bufferedreader.readline method throws IOException), Buffer Edreader will be closed.

Before Java SE 7, blocks can be used finally to ensure that resources are shut down, whether the try statements are completed properly or if they occur unexpectedly. The following example finally replaces the-with-resources statement with a block try :

1     staticthrows  IOException {2       new BufferedReader (new  FileReader (path)); 3       try  {4         return  br.readline (); 5       finally  {6         ifnull) br.close (); 7       }8     }

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However, in this example, if the readLine and close method throws an exception, then the readFirstLineFromFileWithFinallyBlock method throws finally an exception thrown from the block; trythe exception thrown in the block is suppressed.  Conversely, readFirstLineFromFile 这个例子中 if try both the block and the try -with-resources statement throw an exception, the exception thrown readFirstLineFromFile from try the block is thrown; trythe exception thrown by the-with-resources block is suppressed. In Java SE 7 and later, you can retrieve suppressed exceptions, see suppressed Exceptions for details.

You can try declare one or more resources in a-with-resources statement . The following example retrieves the names of all the files in the zip file zipFileName and creates a text file that contains those file names:

1      Public Static voidwritetofilezipfilecontents (String zipfilename, string outputfilename)2       throwsjava.io.IOException {3 4Java.nio.charset.Charset charset = java.nio.charset.Charset.forName ("Us-ascii");5Java.nio.file.Path Outputfilepath =Java.nio.file.Paths.get (outputfilename);6 7       //Open zip file and create output file with Try-with-resources statement8 9       Try (TenJava.util.zip.ZipFile ZF =NewJava.util.zip.ZipFile (zipfilename); OneJava.io.BufferedWriter writer =Java.nio.file.Files.newBufferedWriter (Outputfilepath, CharSet) A       ) { -  -         //Enumerate each entry the  -          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; AWriter.write (zipentryname, 0, Zipentryname.length ()); at         } -       } -}

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In this example, the try -with-resources statement contains two declarations separated by semicolons: ZipFile and BufferedWriter . When the code block is directly accompanied by it normally or due to an abnormal termination, BufferedWriter and the ZipFile 对象的 close method is called automatically in this order. Note: The method invocation order of the resources is the close opposite of the order in which they were created.

The following example uses a try -with-resources statement to automatically close an java.sql.Statement object:

1      Public Static voidViewtable (Connection con)throwsSQLException {2 3String query = "Select Cof_name, sup_id, Price, SALES, total from coffees";4 5       Try(Statement stmt =con.createstatement ()) {6 7ResultSet rs =stmt.executequery (query);8 9          while(Rs.next ()) {TenString coffeename = rs.getstring ("Cof_name"); One           intSupplierID = Rs.getint ("sup_id"); A           floatPrice = Rs.getfloat ("Price"); -           intSales = Rs.getint ("Sales"); -           intTotal = Rs.getint ("Total"); theSystem.out.println (Coffeename + "," + SupplierID + "," + Price + -"," + Sales + "," +Total ); -         } -  +}Catch(SQLException e) { - jdbctutorialutilities.printsqlexception (e); +       } A}

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This resource used in this example java.sql.Statement is part of JDBC 4.1 and subsequent versions of the API.

Note : A try -with-resources statement can try have and block like a normal catch statement finally . In the try -with-resources statement, any catch or all of the finally blocks run after the declared resource is closed.

Suppressed anomalies

trythe code block associated with the-with-resources statement may throw an exception. When writeToFileZipFileContents这个例子中 attempting to close ZipFile and BufferedWriter object, the try block may throw an exception, and the try -with-resources statement may throw up to two exceptions. If the try block throws an exception and the try -with-resources statement throws one or more exceptions, the try exception thrown from the-with-resources statement is suppressed, and the exception thrown by the block is the writeToFileZipFileContents one thrown by the method. You can try块抛出的异常的 Throwable.getSuppressed retrieve these suppressed exception information by calling the method .

Class that implements the Autocloseable or Closeable interface

See also the Javadoc of the AutoCloseable Closeable interface to see the set of classes that implement either of the two interfaces . Closeable interface inherits the interface AutoCloseable . The method of the interface Closeable close throws a IOException type of exception and the method of the AutoCloseable interface close throws so that the subclass of the subclasses of the Exception 类型的异常。 AutoCloseable interface can override the close method of this Behavior to throw the specified exception, for example IOException , or no exception.

This document is translated from the Oracle official documentation http://docs.oracle.com/javase/7/docs/technotes/guides/language/try-with-resources.html, if there is an incorrect place , please correct me, thank you!

Java NIO try-with-resources [reprint]