眾所周知，所有被打開的系統資源，比如流、文件或者Socket連接等，都需要被開發者手動關閉，否則隨著程序的不斷運行，資源泄露將會累積成重大的生產事故。 在Java的江湖中，存在著一種名為finally的功夫，它可以保證當你習武走火入魔之時，還可以做一些自救的操作。在遠古時代，處理資源關閉的代碼通常寫在finally塊中。然而，如果你同時打開了多個資源，那么將會出現噩夢般的場景： ``` public class Demo { public static void main(String[] args) { BufferedInputStream bin = null; BufferedOutputStream bout = null; try { bin = new BufferedInputStream(new FileInputStream(new File("test.txt"))); bout = new BufferedOutputStream(new FileOutputStream(new File("out.txt"))); int b; while ((b = bin.read()) != -1) { bout.write(b); } } catch (IOException e) { e.printStackTrace(); } finally { if (bin != null) { try { bin.close(); } catch (IOException e) { e.printStackTrace(); } finally { if (bout != null) { try { bout.close(); } catch (IOException e) { e.printStackTrace(); } } } } } } } ``` 我們用try-with-resource來改寫剛才的例子： ``` public class TryWithResource { public static void main(String[] args) { try (BufferedInputStream bin = new BufferedInputStream(new FileInputStream(new File("test.txt"))); BufferedOutputStream bout = new BufferedOutputStream(new FileOutputStream(new File("out.txt")))) { int b; while ((b = bin.read()) != -1) { bout.write(b); } } catch (IOException e) { e.printStackTrace(); } } } ``` 示例： ``` public class AppTest { public static void main(String[] args){ try(Connection con = new Connection()) { con.print(); }catch (Exception e){ e.printStackTrace(); } } static class Connection implements AutoCloseable{ public void print(){ System.err.println("print data==>"); } @Override public void close() throws Exception { System.err.println("auto close===>"); } } } 語法糖原理： public class AppTest { public AppTest() { } public static void main(String[] args) { try { AppTest.Connection con = new AppTest.Connection(); Throwable var2 = null; try { con.print(); } catch (Throwable var12) { var2 = var12; throw var12; } finally { if (con != null) { if (var2 != null) { try { con.close(); } catch (Throwable var11) { var2.addSuppressed(var11); } } else { con.close(); } } } } catch (Exception var14) { var14.printStackTrace(); } } static class Connection implements AutoCloseable { Connection() { } public void print() { System.err.println("print data==>"); } public void close() throws Exception { System.err.println("auto close===>"); } } } ```