1.并發與并行

個人理解并發就是在時間上運行程序（即不同任務在不同時間片上運行），并行就是在空間上運行程序（即不同任務在不同處理器或計算機上運行）。

2.Java中的Thread類

(1)Thread類通過實現Runnable接口

(2)線程池(ThreadPool)用來管理線程的數量

我們先用一個例子實現:

• 創建并啟動100個線程，每個線程都往同一個賬戶添加一元。
• 定義一個名為Account類模擬賬戶，一個名為AddAYuanTask的類用來向賬戶里添加一元。

程序如下

第一版Account
import?java.util.concurrent.*;
public?class?AccountWithoutSync?{
????private?static?Account?account?=?new?Account();
????public?static?void?main(String[]?args)
????{
????????ExecutorService?executor?=?Executors.newCachedThreadPool();
????????for(int?i?=?0;?i?<?100;?i++)
????????{
????????????executor.execute(new?AddOneYuanTask());
????????}
????????executor.shutdown();?
????????while(!executor.isTerminated())
????????{?????
????????}?
????????System.out.println("What?is?balance??"?+?account.getBalance());
????}
????
????//Inner?class
????private?static?class?AddOneYuanTask?implements?Runnable
????{
????????public?void?run()
????????{
????????????account.deposit(1);
????????}
????}
????
????private?static?class?Account
????{
????????private?int?balance?=?0;
????????
????????public?int?getBalance()
????????{
????????????return?balance;
????????}
????????
????????public?void?deposit(int?amount)
????????{
????????????int?newBalance?=?balance?+?amount;
????????????
????????????????????????//人為地制造延時??
????????????try
????????????{
????????????????Thread.sleep(5);
????????????}
????????????catch(InterruptedException?ex)
????????????{
????????????}
????????????
????????????balance?=?newBalance;
????????}
????}
}

我們運行一下發現balance為4或5，這是個錯誤的結果，如果一個類的對象在多線程程序中導致競爭狀態，則稱這個類為線程不安全的。所以這個任務是線程不安全的。

3.用同步完善程序

用互斥鎖來實現同步，即在一任務開始執行時加鎖，執行完畢后釋放鎖。在釋放鎖之前其它任務無法執行。同步完全可以避免競爭狀態的產生，但有的時候還需要線程之間的相互合作。

然后增加一個向賬戶提款(Withdraw)的任務，當余額小于取款數時，等待新存入的存款。

Account類添加

privatestaticLocklock=newReentrantLock();//創建一個鎖

privatestaticConditionnewDeposit=lock.newCondition();//實現一個條件

Account類中應用互斥鎖的的方法如下

withdraw和deposit

然后程序相應的修改修改

?第二版Account
????public?static?void?main(String[]?args)
????{
????????System.out.println("Thread?1\t\tThread?2\t\tBalance");
????????
????????ExecutorService?executor?=?Executors.newFixedThreadPool(2);
????????executor.execute(new?DepositTask());
????????executor.execute(new?WithdrawTask());
????????executor.shutdown();
????}
????
????
????public?static?class?DepositTask?implements?Runnable
????{
????????public?void?run()
????????{
????????????try
????????????{
????????????????while(true)
????????????????{
????????????????????account.deposit((int)(Math.random()?*?10)?+?1);
????????????????????Thread.sleep(1000);
????????????????}
????????????}
????????????catch(InterruptedException?ex)
????????????{
????????????????ex.printStackTrace();
????????????}
????????}
????}
????
????public?static?class?WithdrawTask?implements?Runnable
????{
????????public?void?run()
????????{
????????????while(true)
????????????{
????????????????account.withdraw((int)(Math.random()?*?10)?+?1);
????????????}
????????}
????}

4.客戶端/服務器的網絡應用

Java中對socket的使用十分方便，在建立socket連接后就可以使用輸入輸出流的方法實現數據傳輸了。

在實現基本的GUI后，在服務器用一個判斷條件永遠為true的循環來監聽客戶端的連接請求(Socketsocket=serverSocket.accept();

服務器通過創建一個內部類(HandleAClient)，把客服端的socket傳遞過來執行。

HandleAClient類
????class?HandleAClient?implements?Runnable
????{
????????//A?connected?socket
????????private?Socket?socket;
????????
????????/**Construct?a?thread?*/
????????public?HandleAClient(Socket?socket)
????????{
????????????this.socket?=?socket;
????????}
????????
????????/**Run?a?thread?*/
????????public?void?run()
????????{
????????????try
????????????{
????????????????//Create?data?input?and?output?streams
????????????????DataInputStream?inputFromClient?=?new?DataInputStream(
????????????????????????socket.getInputStream());
????????????????DataOutputStream?outputToClient?=?new?DataOutputStream(
????????????????????????socket.getOutputStream());
????????????????int?order?=?0;
????????????????double?amount;
????????????????//Continuously?serve?the?client
????????????????while(order?!=?4)
????????????????{
????????????????????//Receive?order,?amount?from?the?client
????????????????????order?=?inputFromClient.readInt();
????????????????????amount?=?inputFromClient.readDouble();
????????????????????
????????????????????if(order?==?1)
????????????????????{
????????????????????????outputToClient.writeDouble(account.getBalance());
????????????????????}
????????????????????else?if(order?==?2)
????????????????????{
????????????????????????account.withdraw(amount);
????????????????????????outputToClient.writeDouble(account.getBalance());
????????????????????}
????????????????????else?if(order?==?3)
????????????????????{
????????????????????????account.deposit(amount);
????????????????????????outputToClient.writeDouble(account.getBalance());
????????????????????}
????????????????????
????????????????????jta.append("Order?received?from?client:?"+
????????????????????????????order?+?'\n');
????????????????????jta.append("Balance?is?"?+?account.getBalance()?+?'\n');
????????????????}
????????????}
????????????catch(IOException?e)
????????????{
????????????????System.err.println(e);
????????????}
????????}
????}

而客戶端連接上服務器后，創建兩個IO流

? //IOstreams
? ? DataOutputStreamtoServer=newDataOutputStream(socket.getOutputStream());
? ? DataInputStreamfromServer=newDataInputStream(socket.getInputStream());
? ? 即可傳輸order(菜單選項)和amount
? ? //Sendtheorder,amounttotheserver
? ? toServer.writeInt(order);
? ? toServer.writeDouble(amount);
? ? toServer.flush();
? ? 最后再讀取balance
? ? balance=fromServer.readDouble();

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