jdk1.7和jdk1.8區別

轉自：http://www.2cto.com/kf/201307/225968.html

本文是我學習瞭解了jdk7和jdk8的一些新特性的一些資料，有興趣的大家可以瀏覽下下面的內容。

官方文件：http://www.oracle.com/technetwork/java/javase/jdk7-relnotes-418459.html

在jdk7的新特性方面主要有下面幾方面的增強：

1.jdk7語法上

   1.1二進位制變數的表示,支援將整數型別用二進位制來表示，用0b開頭。

   // 所有整數 int， short，long，byte都可以用二進位制表示
    // 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;

    // 二進位制在陣列等的使用

    final int[] phases = { 0b00110001, 0b01100010, 0b11000100, 0b10001001,
    0b00010011, 0b00100110, 0b01001100, 0b10011000 };

1.2 Switch語句支援string型別

       public static String getTypeOfDayWithSwitchStatement(String dayOfWeekArg) {
         String typeOfDay;
         switch (dayOfWeekArg) {
             case "Monday":
                 typeOfDay = "Start of work week";

                 break;
             case "Tuesday":
             case "Wednesday":
             case "Thursday":
                 typeOfDay = "Midweek";
                 break;
             case "Friday":
                 typeOfDay = "End of work week";
                 break;
             case "Saturday":
             case "Sunday":
                 typeOfDay = "Weekend";
                 break;
             default:
                 throw new IllegalArgumentException("Invalid day of the week: " + dayOfWeekArg);
         }
         return typeOfDay;
    }

1.3 Try-with-resource語句

注意：實現java.lang.AutoCloseable介面的資源都可以放到try中，跟final裡面的關閉資源類似；按照宣告逆序關閉資源 ;Try塊丟擲的異常通過Throwable.getSuppressed獲取

    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多個異常說明：Catch異常型別為final；生成Bytecode 會比多個catch小； Rethrow時保持異常型別

    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 數字型別的下劃線表示更友好的表示方式，不過要注意下劃線新增的一些標準，可以參考下面的示例

    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 = 5_______2;        // 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 泛型例項的建立可以通過型別推斷來簡化可以去掉後面new部分的泛型型別，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>>>();

    //編譯器使用尖括號 (<>) 推斷型別
    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在可變引數方法中傳遞非具體化引數,改進編譯警告和錯誤

Heap pollution 指一個變數被指向另外一個不是相同型別的變數。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);
    }
    }
    你會得到一個warning
    warning: [varargs] Possible heap pollution from parameterized vararg type
    要消除警告，可以有三種方式
    1.加 annotation @SafeVarargs
    2.加 annotation @SuppressWarnings({"unchecked", "varargs"})
    3.使用編譯器引數 –Xlint:varargs;

1.8 資訊更豐富的回溯追蹤就是上面try中try語句和裡面的語句同時丟擲異常時，異常棧的資訊

    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)

2. NIO2的一些新特性

    1.java.nio.file 和java.nio.file.attribute包支援更詳細屬性，比如許可權，所有者
    2. symbolic and hard links支援
    3. Path訪問檔案系統，Files支援各種檔案操作
    4.高效的訪問metadata資訊
    5.遞迴查詢檔案樹，檔案擴充套件搜尋
    6.檔案系統修改通知機制
    7.File類操作API相容
    8.檔案隨機訪問增強 mapping a region,locl a region,絕對位置讀取
    9. AIO Reactor（基於事件）和Proactor

下面列一些示例：

2.1IO and New IO 監聽檔案系統變化通知

通過FileSystems.getDefault().newWatchService()獲取watchService，然後將需要監聽的path目錄註冊到這個watchservice中，對於這個目錄的檔案修改，新增，刪除等實踐可以配置，然後就自動能監聽到響應的事件。

    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遍歷檔案樹，通過繼承SimpleFileVisitor類，實現事件遍歷目錄樹的操作，然後通過Files.walkFileTree(listDir, opts, Integer.MAX_VALUE, walk);這個API來遍歷目錄樹

    private void workFilePath() {
    Path listDir = Paths.get("/tmp"); // define the starting file
    ListTree walk = new ListTree();
    …Files.walkFileTree(listDir, walk);…
    // 遍歷的時候跟蹤連結
    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 遞迴遍歷檔案目錄的介面
    @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非同步IO 檔案和網路非同步IO在java
NIO2實現了，都是用AsynchronousFileChannel，AsynchronousSocketChanne等實現，關於同步阻塞IO，同步非阻塞IO，非同步阻塞IO和非同步非阻塞IO在ppt的這頁上下面備註有說明，有興趣的可以深入瞭解下。Java NIO2中就實現了作業系統的非同步非阻塞IO。

    // 使用AsynchronousFileChannel.open(path, withOptions(),
        // taskExecutor))這個API對非同步檔案IO的處理
        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
    ……

3. JDBC 4.1

3.1.可以使用try-with-resources自動關閉Connection, ResultSet, 和 Statement資源物件

3.2. RowSet 1.1：引入RowSetFactory介面和RowSetProvider類，可以建立JDBC driver支援的各種 row sets，這裡的rowset實現其實就是將sql語句上的一些操作轉為方法的操作，封裝了一些功能。

3.3. JDBC-ODBC驅動會在jdk8中刪除

    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

4. 併發工具增強

4.1.fork-join
最大的增強，充分利用多核特性，將大問題分解成各個子問題，由多個cpu可以同時解決多個子問題，最後合併結果，繼承RecursiveTask，實現compute方法，然後呼叫fork計算，最後用join合併結果。

    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 併發下隨機數生成類，保證併發下的隨機數生成的執行緒安全，實際上就是使用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();

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