JDK7新特性，你知道幾個？

時間  2021-01-02
標籤 java sql 數組 安全 網絡 併發 app dom 異步 async 欄目 Java 简体版
原文   原文鏈接

前言

以前學習的過程當中，天天都是老師說這個是JDK7之後可使用，那個是JDK8之後可使用，天天都記的很混亂，今天專門忙裏偷閒，歸攏整理下JDK7的新特性，對於JDK的新特性，後期會進行整理更新，但願能夠幫助到有須要的朋友。java

一，語法上

1.1switch中可使用字串了

String s = "test";
switch (s) {
 case "test" :
   System.out.println("test");
case "test1" :
   System.out.println("test1");
break ;
default :
  System.out.println("break");
break ;
}

1.2二進制變量的表示,支持將整數類型用二進制來表示，用0b開頭。

// 全部整數 int， short，long，byte均可以用二進制表示
// 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;
// 二進制在數組等的使用
final int[] phases = { 0b00110001, 0b01100010, 0b11000100, 0b10001001,0b00010011, 0b00100110, 0b01001100, 0b10011000 };

1.3 Try-with-resource語句

注意：實現java.lang.AutoCloseable接口的資源均可以放到try中，跟final裏面的關閉資源相似； 按照聲明逆序關閉資源 ;Try塊拋出的異常經過Throwable.getSuppressed獲取sql

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異常類型爲final； 生成Bytecode 會比多個catch小； Rethrow時保持異常類型

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 數字類型的下劃線表示 更友好的表示方式，不過要注意下劃線添加的一些標準，能夠參考下面的示例

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 = 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 泛型實例的建立能夠經過類型推斷來簡化 能夠去掉後面new部分的泛型類型，只用<>就能夠了。

//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 (<>) 推斷類型
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在可變參數方法中傳遞非具體化參數,改進編譯警告和錯誤

Heap pollution 指一個變量被指向另一個不是相同類型的變量。例如數組

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);
}
}
你會獲得一個warning
warning: [varargs] Possible heap pollution from parameterized vararg type
要消除警告，能夠有三種方式
1.加 annotation @SafeVarargs
2.加 annotation @SuppressWarnings({"unchecked", "varargs"})
3.使用編譯器參數 –Xlint:varargs;

1.8 信息更豐富的回溯追蹤 就是上面try中try語句和裏面的語句同時拋出異常時，異常棧的信息

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)

2、NIO2的一些新特性

  1. java.nio.file 和java.nio.file.attribute包 支持更詳細屬性，好比權限，全部者
  2. symbolic and hard links支持
  3. Path訪問文件系統，Files支持各類文件操做
  4. 高效的訪問metadata信息
  5. 遞歸查找文件樹，文件擴展搜索
  6. 文件系統修改通知機制
  7. File類操做API兼容
  8. 文件隨機訪問加強 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：

    2.1 IO and New IO 監聽文件系統變化通知

    經過FileSystems.getDefault().newWatchService()獲取watchService，而後將須要監聽的path目錄註冊到這個watchservice中，對於這個目錄的文件修改，新增，刪除等實踐能夠配置，而後就自動能監聽到響應的事件。安全

    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遍歷文件樹，經過繼承SimpleFileVisitor類，實現事件遍歷目錄樹操做，而後經過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 遞歸遍歷文件目錄的接口
@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異步IO 文件和網絡 異步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，異步阻塞IO和異步非阻塞IO在ppt的這頁上下面備註有說明，有興趣的能夠深刻了解下。Java NIO2中就實現了操做系統的異步非阻塞IO。網絡

    // 使用AsynchronousFileChannel.open(path, withOptions(),taskExecutor))這個API對異步文件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)
……

    三. JDBC 4.1

3.1.可使用try-with-resources自動關閉Connection, ResultSet, 和 Statement資源對象
3.2. RowSet 1.1：引入RowSetFactory接口和RowSetProvider類，能夠建立JDBC driver支持的各類 row sets，這裏的rowset實現其實就是將sql語句上的一些操做轉爲方法的操做，封裝了一些功能。
3.3. JDBC-ODBC驅動會在jdk8中刪除併發

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、併發工具加強

4.1.fork-join

最大的加強，充分利用多核特性，將大問題分解成各個子問題，由多個cpu能夠同時解決多個子問題，最後合併結果，繼承RecursiveTask，實現compute方法，而後調用fork計算，最後用join合併結果。app

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線程安全，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();
}
long end2 = System.nanoTime() - start2;
System.out.println("use time2 : " + end2);

4.3. phaser 相似cyclebarrier和countdownlatch，不過能夠動態添加資源減小資源

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();
}

結尾

因爲我的能力問題，還有不少須要改進的地方，若有錯誤，煩請各位大佬私信或留言進行批評指正，在下定當及時更正！dom

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