JDK7主要特性介绍

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jdk7&8 的架构图

JDK7新增特性

1、Java 编程语言特性

• 1.1二进制数字表达方式

  the integral types (byte, short, int, and long) can also be expressed using the binary number system。例如：

  byte aByte = (byte)0b00; // An 8-bit 'byte' value short aShort = (short)0b00101; // A 16-bit 'short' value: int anInt1 = 0b000; // Some 32-bit 'int' values int anInt2 = 0b101; int anInt3 = 0B101; // The B can be upper or lower case.

  1.2 使用下划线对数字进行分隔表达，例如可表示： 1_322_222

• 1.3 switch 语句支持字符串变量，例如：switch (String str) {…}
• 1.4 泛型实例创建的类型推断,例如： 使用可变参数时，提升编译器的警告和错误信息

  Map<String, List<String>> myMap = new HashMap<>();// an empty set of type parameters (<>) to init constructor. List<? extends String> list2 = new ArrayList<>(); list.addAll(list2);

• 1.5 try-with-resources 语句

  说明：The try-with-resources statement is a try statement that declares one or more resources. A resource is as an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource

  格式：

  try(// open resources here){     // use resources } catch (FileNotFoundException e) {     // exception handling } // resources are closed as soon as try-catch block is executed. 

  优点：

  • 代码的可读性更好，更易于编写代码
  • 自动化的资源管理
  • 代码的行数被减少
  • 不需要使用finally块来关闭资源

我们可以通过在try-with-resources中使用分号来间隔多个资源，例子:

try ( BufferedReader br = new BufferedReader(new FileReader("C:\\python.txt")); java.io.BufferedWriter writer=java.nio.file.Files. newBufferedWriter(FileSystems.getDefault().getPath("C:\\journaldev.txt"), Charset.defaultCharset())) { System.out.println(br.readLine()); } catch (IOException e){ e.printStackTrace(); } 

1.6 同时捕获多个异常处理    1）Handling More Than One Type of Exception：a single catch block can handle more than one type of exception，例如：catch (IOException|SQLException ex) {}     2）Rethrowing Exceptions with More Inclusive Type Checking：rethrow exception allows you to specify more specific exception types in the throws clause of a method declaration。例如：

package com.journaldev.util; public class Java7MultipleExceptions { public static void main(String[] args) { try{ rethrow("abc"); }catch(FirstException | SecondException | ThirdException e){ //below assignment will throw compile time exception since e is final //e = new Exception(); System.out.println(e.getMessage()); } } static void rethrow(String s) throws FirstException, SecondException,ThirdException { try { if (s.equals("First")) throw new FirstException("First"); else if (s.equals("Second")) throw new SecondException("Second"); else throw new ThirdException("Third"); } catch (Exception e) { //below assignment disables the improved rethrow exception type checking feature of Java 7 // e=new ThirdException(); throw e; } } static class FirstException extends Exception { public FirstException(String msg) { super(msg); } } ....... static class ThirdException extends Exception { public ThirdException(String msg) { super(msg); } } }

2、并发

2.1 fork/join 框架（分而治之框架）

    Fork 原始含义叉子、分叉的意思。在Linux 平台中，函数 fork()用来创建子进程，使得系统进程可以多一个执行分支; Join表示等待。fork()后系统多一个执行分支-线程，需要等待此执行分支执行完毕，才有可能得到结果，因此 join 就是表示等待。

   实际使用中，如果大量使用 fork 开启线程进行处理，那么可能导致系统由过多线程而严重影响性能。故JDK给出ForkJoinPool 线程池，将fork开启线程交给ForkJoiinPool 线程池进行处理，节省系统资源。ForkJoinPool来支持将一个任务拆分成多个“小任务”并行计算，再把多个“小任务”的结果合并成总的计算结果。

   ForkJoinPool是ExecutorService的实现类，因此是一种特殊的线程池。

   使用方法：1)初始化 ForkJoinPool commonPool =  ForkJoinPool.commonPool(); 2）执行指定任务commonPool.submit(ForkJoinTask<T> task) 或invoke(ForkJoinTask<T> task)。ForkJoinTask代表并行、合并的任务;ForkJoinTask是一个抽象类，它还有两个抽象子类：RecusiveAction(返回值任务)和RecusiveTask(无返回值任务）

    RecusiveAction(返回值任务)和RecusiveTask(无返回值任务） Demo见：https://www.cnblogs.com/lixuwu/p/7979480.html

2.4 IO

2.4.1 递归查找文件树

Files.walkFileTree(Path filepath,class T extends SimpleFileVisitor<Path>)

  DEMO：

import java.io.IOException; import java.nio.file.FileVisitResult; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.nio.file.SimpleFileVisitor; import java.nio.file.attribute.BasicFileAttributes; import java.util.regex.Matcher; import java.util.regex.Pattern; / * @Description : 遍历目录树中的.java .class程序文件 */ public class Test { public static void main(String[] args) throws IOException { Path path = Paths.get(""); System.out.println(path.toAbsolutePath()); String regex = "^.+?\\.(?:(?:java)||(?:class))$"; final Pattern pattern = Pattern.compile(regex); Files.walkFileTree(path, new SimpleFileVisitor<Path>() { public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException { String f = file.toString(); Matcher m = pattern.matcher(f); if (m.matches()) { System.out.println(f); } return FileVisitResult.CONTINUE; } }); } }

2.4.2 文件系统修改通知

import java.nio.file.Path; import java.nio.file.Paths; import java.nio.file.WatchEvent; import java.nio.file.WatchKey; import java.nio.file.WatchService; import static java.nio.file.StandardWatchEventKind.*; / *JDK7 文件系统修改通知机制 */ public class TestWatcherService { private WatchService watcher; public TestWatcherService(Path path)throws IOException{ watcher = FileSystems.getDefault().newWatchService(); //1 newWatchService方法实例化WatchService监听服务 path.register(watcher, ENTRY_CREATE,ENTRY_DELETE,ENTRY_MODIFY); //2 目录注册，watcher监听目录操作事件创建、删除、修改 } public void handleEvents() throws InterruptedException{ while(true){ WatchKey key = watcher.take(); for(WatchEvent<?> event : key.pollEvents()){ WatchEvent.Kind kind = event.kind(); //3 获取事件Event，Event :ENTRY_CREATE,ENTRY_DELETE,ENTRY_MODIFY if(kind == OVERFLOW){//事件可能lost or discarded continue; } WatchEvent<Path> e = (WatchEvent<Path>)event; Path fileName = e.context(); System.out.printf("Event %s has happened, which fileName is %s%n" ,kind.name(),fileName); } if(!key.reset()){ break; } } } }

PS:https://www.journaldev.com/588/java-switch-case-string