JDK7新特性,你知道几个?
前言
之前学习的过程中,每天都是老师说这个是JDK7以后可以使用,那个是JDK8以后可以使用,每天都记的很混乱,今天专门忙里偷闲,归拢整理下JDK7的新特性,对于JDK的新特性,后期会进行整理更新,希望可以帮助到有需要的朋友。
一,语法上
1.1switch中可以使用字串了
String s = "test"; switch (s) { case "test" : System.out.println("test"); case "test1" : System.out.println("test1"); break ; default : System.out.println("break"); break ; }
1.2二进制变量的表示,支持将整数类型用二进制来表示,用0b开头。
// 所有整数 int, short,long,byte都可以用二进制表示 // An 8-bit 'byte' value: byte aByte = (byte) 0b00100001; // A 16-bit 'short' value: short aShort = (short) 0b1010000101000101; // Some 32-bit 'int' values: intanInt1 = 0b10100001010001011010000101000101; intanInt2 = 0b101; intanInt3 = 0B101; // The B can be upper or lower case. // A 64-bit 'long' value. Note the "L" suffix: long aLong = 0b1010000101000101101000010100010110100001010001011010000101000101L; // 二进制在数组等的使用 final int[] phases = { 0b00110001, 0b01100010, 0b11000100, 0b10001001,0b00010011, 0b00100110, 0b01001100, 0b10011000 };
1.3 Try-with-resource语句
注意:实现java.lang.AutoCloseable接口的资源都可以放到try中,跟final里面的关闭资源类似; 按照声明逆序关闭资源 ;Try块抛出的异常通过Throwable.getSuppressed获取
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()); } } }
1.4 Catch多个异常 说明:Catch异常类型为final; 生成Bytecode 会比多个catch小; Rethrow时保持异常类型
public static void main(String[] args) throws Exception { try { testthrows(); } catch (IOException | SQLException ex) { throw ex; } } public static void testthrows() throws IOException, SQLException {}
1.5 数字类型的下划线表示 更友好的表示方式,不过要注意下划线添加的一些标准,可以参考下面的示例
long creditCardNumber = 1234_5678_9012_3456L; long socialSecurityNumber = 999_99_9999L; float pi = 3.14_15F; long hexBytes = 0xFF_EC_DE_5E; long hexWords = 0xCAFE_BABE; long maxLong = 0x7fff_ffff_ffff_ffffL; byte nybbles = 0b0010_0101; long bytes = 0b11010010_01101001_10010100_10010010; //float pi1 = 3_.1415F;// Invalid; cannot put underscores adjacent to a decimal point //float pi2 = 3._1415F;// Invalid; cannot put underscores adjacent to a decimal point //long socialSecurityNumber1= 999_99_9999_L;// Invalid; cannot put underscores prior to an L suffix //int x1 = _52;// This is an identifier, not a numeric literal int x2 = 5_2;// OK (decimal literal) //int x3 = 52_;// Invalid; cannot put underscores at the end of a literal int x4 = 52;// OK (decimal literal) //int x5 = 0_x52;// Invalid; cannot put underscores in the 0x radix prefix //int x6 = 0x_52;// Invalid; cannot put underscores at the beginning of a number int x7 = 0x5_2;// OK (hexadecimal literal) //int x8 = 0x52_;// Invalid; cannot put underscores at the end of a number int x9 = 0_52; // OK (octal literal) int x10 = 05_2;// OK (octal literal) //int x11 = 052_;// Invalid; cannot put underscores at the end of a number
1.6 泛型实例的创建可以通过类型推断来简化 可以去掉后面new部分的泛型类型,只用<>就可以了。
//使用泛型前 List strList = new ArrayList(); List<String> strList4 = new ArrayList<String>(); List<Map<String, List<String>>> strList5 = new ArrayList<Map<String, List<String>>>(); //编译器使用尖括号 (<>) 推断类型 List<String> strList0 = new ArrayList<String>(); List<Map<String, List<String>>> strList1 = new ArrayList<Map<String, List<String>>>(); List<String> strList2 = new ArrayList<>(); List<Map<String, List<String>>> strList3 = new ArrayList<>(); List<String> list = new ArrayList<>(); list.add("A"); // The following statement should fail since addAll expects // Collection<? extends String> //list.addAll(new ArrayList<>());
1.7在可变参数方法中传递非具体化参数,改进编译警告和错误
Heap pollution 指一个变量被指向另外一个不是相同类型的变量。例如
List l = new ArrayList<Number>(); List<String> ls = l; // unchecked warning l.add(0, new Integer(42)); // another unchecked warning String s = ls.get(0); // ClassCastException is thrown Jdk7: public static <T> void addToList (List<T> listArg, T... elements) { for (T x : elements) { listArg.add(x); } } 你会得到一个warning warning: [varargs] Possible heap pollution from parameterized vararg type 要消除警告,可以有三种方式 1.加 annotation @SafeVarargs 2.加 annotation @SuppressWarnings({"unchecked", "varargs"}) 3.使用编译器参数 –Xlint:varargs;
1.8 信息更丰富的回溯追踪 就是上面try中try语句和里面的语句同时抛出异常时,异常栈的信息
java.io.IOException at Suppress.write(Suppress.java:19)at Suppress.main(Suppress.java:8) Suppressed: java.io.IOException at Suppress.close(Suppress.java:24) at Suppress.main(Suppress.java:9) Suppressed: java.io.IOException at Suppress.close(Suppress.java:24) at Suppress.main(Suppress.java:9)
二、NIO2的一些新特性
- java.nio.file 和java.nio.file.attribute包 支持更详细属性,比如权限,所有者
- symbolic and hard links支持
- Path访问文件系统,Files支持各种文件操作
- 高效的访问metadata信息
- 递归查找文件树,文件扩展搜索
- 文件系统修改通知机制
- File类操作API兼容
- 文件随机访问增强 mapping a region,locl a region,绝对位置读取
- AIO Reactor(基于事件)和Proactor
下面列一些示例:2.1 IO and New IO 监听文件系统变化通知
通过FileSystems.getDefault().newWatchService()获取watchService,然后将需要监听的path目录注册到这个watchservice中,对于这个目录的文件修改,新增,删除等实践可以配置,然后就自动能监听到响应的事件。
private WatchService watcher; public TestWatcherService(Path path) throws IOException { watcher = FileSystems.getDefault().newWatchService(); path.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY); } public void handleEvents() throws InterruptedException { while (true) { WatchKey key = watcher.take(); for (WatchEvent<?> event : key.pollEvents()) { WatchEvent.Kind kind = event.kind(); 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; }
2.2 IO and New IO遍历文件树,通过继承SimpleFileVisitor类,实现事件遍历目录树操作,然后通过Files.walkFileTree(listDir, opts,Integer.MAX_VALUE, walk);这个API来遍历目录树
private void workFilePath() { Path listDir = Paths.get("/tmp"); // define the starting file ListTree walk = new ListTree(); Files.walkFileTree(listDir, walk); // 遍历的时候跟踪链接 EnumSet opts = EnumSet.of(FileVisitOption.FOLLOW_LINKS); try { Files.walkFileTree(listDir, opts, Integer.MAX_VALUE, walk); } catch (IOException e) { System.err.println(e); } class ListTree extends SimpleFileVisitor<Path> {// NIO2 递归遍历文件目录的接口 @Override public FileVisitResult postVisitDirectory(Path dir, IOException exc) { System.out.println("Visited directory: " + dir.toString()); return FileVisitResult.CONTINUE; } @Override public FileVisitResult visitFileFailed(Path file, IOException exc) { System.out.println(exc); return FileVisitResult.CONTINUE; } }
2.3 AIO异步IO 文件和网络 异步IO在java
NIO2实现了,都是用AsynchronousFileChannel,AsynchronousSocketChanne等实现,关于同步阻塞IO,同步非阻塞IO,异步阻塞IO和异步非阻塞IO在ppt的这页上下面备注有说明,有兴趣的可以深入了解下。Java NIO2中就实现了操作系统的异步非阻塞IO。
// 使用AsynchronousFileChannel.open(path, withOptions(),taskExecutor))这个API对异步文件IO的处理 public static void asyFileChannel2() { final int THREADS = 5; ExecutorService taskExecutor = Executors.newFixedThreadPool(THREADS); String encoding = System.getProperty("file.encoding"); List<Future<ByteBuffer>> list = new ArrayList<>(); int sheeps = 0; Path path = Paths.get("/tmp","store.txt"); try (AsynchronousFileChannel asynchronousFileChannel = AsynchronousFileChannel.open(path, withOptions(), taskExecutor)) { for (int i = 0; i < 50; i++) { Callable<ByteBuffer> worker = new Callable<ByteBuffer>(){ @Override public ByteBuffer call() throws Exception { ByteBuffer buffer = ByteBuffer.allocateDirect(ThreadLocalRandom.current().nextInt(100, 200)); asynchronousFileChannel.read(buffer, ThreadLocalRandom) ……
三. JDBC 4.1
3.1.可以使用try-with-resources自动关闭Connection, ResultSet, 和 Statement资源对象
3.2. RowSet 1.1:引入RowSetFactory接口和RowSetProvider类,可以创建JDBC driver支持的各种 row sets,这里的rowset实现其实就是将sql语句上的一些操作转为方法的操作,封装了一些功能。
3.3. JDBC-ODBC驱动会在jdk8中删除
try (Statement stmt = con.createStatement()) { RowSetFactory aFactory = RowSetProvider.newFactory(); CachedRowSet crs = aFactory.createCachedRowSet(); RowSetFactory rsf = RowSetProvider.newFactory("com.sun.rowset.RowSetFactoryImpl", null); WebRowSet wrs = rsf.createWebRowSet(); createCachedRowSet createFilteredRowSet createJdbcRowSet createJoinRowSet createWebRowSet }
四、并发工具增强
4.1.fork-join
最大的增强,充分利用多核特性,将大问题分解成各个子问题,由多个cpu可以同时解决多个子问题,最后合并结果,继承RecursiveTask,实现compute方法,然后调用fork计算,最后用join合并结果。
class Fibonacci extends RecursiveTask<Integer> { final int n; Fibonacci(int n) { this.n = n; } private int compute(int small) { final int[] results = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 }; return results[small]; } public Integer compute() { if (n <= 10) { return compute(n); } Fibonacci f1 = new Fibonacci(n - 1); Fibonacci f2 = new Fibonacci(n - 2); System.out.println("fork new thread for " + (n - 1)); f1.fork(); System.out.println("fork new thread for " + (n - 2)); f2.fork(); return f1.join() + f2.join(); } }
4.2.ThreadLocalRandon 并发下随机数生成类,保证并发下的随机数生成的线程安全,实际上就是使用threadlocal
final int MAX = 100000; ThreadLocalRandom threadLocalRandom = ThreadLocalRandom.current(); long start = System.nanoTime(); for (int i = 0; i < MAX; i++) { threadLocalRandom.nextDouble(); } long end = System.nanoTime() - start; System.out.println("use time1 : " + end); long start2 = System.nanoTime(); for (int i = 0; i < MAX; i++) { Math.random(); } long end2 = System.nanoTime() - start2; System.out.println("use time2 : " + end2);
4.3. phaser 类似cyclebarrier和countdownlatch,不过可以动态添加资源减少资源
void runTasks(List<Runnable> tasks) { final Phaser phaser = new Phaser(1); // "1" to register self // create and start threads for (final Runnable task : tasks) { phaser.register(); new Thread() { public void run() { phaser.arriveAndAwaitAdvance(); // await all creation task.run(); } }.start(); } // allow threads to start and deregister self phaser.arriveAndDeregister(); }
结尾
由于个人能力问题,还有很多需要改进的地方,如有错误,烦请各位大佬私信或留言进行批评指正,在下定当及时更正!
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