線程安全的本質(zhì)體現(xiàn)在兩個(gè)方面，

　　Ａ變量安全：多線程同時(shí)運(yùn)行一段代碼

　　Ｂ線程同步：一個(gè)線程還沒(méi)執(zhí)行完，另一個(gè)線程又進(jìn)來(lái)接著執(zhí)行。

　看個(gè)簡(jiǎn)單的例子。

public class ThreadSafe implements java.lang.Runnable {
int num = 1;
public void run() {
for (int i = 0; i < 3; i++) {
num = num + 1;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
}
}
}

 運(yùn)行結(jié)果

num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------5
num is value +===thread1---------6
num is value +===thread1---------7
num is value +===thread2---------7

很明顯是錯(cuò)誤的，應(yīng)為兩個(gè)線程共享同一個(gè)變量。這里就是變量的安全問(wèn)題。

解決辦法：

１拋棄單實(shí)例，多線程的方式，用多實(shí)例，多線程的方式，這樣就和單線程是一個(gè)樣了，不會(huì)出錯(cuò)，但是是最接近傳統(tǒng)的編程模式

２不要用類的實(shí)例變量，經(jīng)可能把變量封裝到方法內(nèi)部。

１類的解決辦法的代碼。

public class TestMan {
public static void main(String[] args) {
Runnable safe=new ThreadSafe();
Runnable safe2=new ThreadSafe();
Thread thread1=new Thread(safe,"thread1");
Thread thread2=new Thread(safe2,"thread2");
thread1.start();
thread2.start();
}
}

運(yùn)行結(jié)果

num is value +===thread1---------2
num is value +===thread2---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4
２類解決辦法的代碼

public class ThreadSafe implements java.lang.Runnable {
public void run() {
int num = 1;
for (int i = 0; i < 3; i++) {
num = num + 1;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
}
}
}

public class TestMan {
public static void main(String[] args) {
Runnable safe=new ThreadSafe();
Thread thread1=new Thread(safe,"thread1");
Thread thread2=new Thread(safe,"thread2");
thread1.start();
thread2.start();
}
}

 運(yùn)行結(jié)果

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

這兩種辦法，比較推薦適用第二個(gè)辦法，就是把變量經(jīng)可能的封裝到風(fēng)發(fā)內(nèi)部，這樣他們就是線程的私有變量了。另外，從jdk1.2后，推出了 threadlocal　對(duì)象，它作為線程的一個(gè)局部變量，可以為每個(gè)線程創(chuàng)建一個(gè)副本，用來(lái)保存每個(gè)線程的屬性，各是各的，互不干擾。單每個(gè) threadlocal變量只能保存一個(gè)變量，假如有多個(gè)變量要保存，那么就要寫(xiě)多個(gè)threadlocal對(duì)象。

我們把代碼改寫(xiě)一下。

public class ThreadSafe implements java.lang.Runnable {
ThreadLocal<Integer> local=new ThreadLocal<Integer>();
public void run() {
for (int i = 0; i < 3; i++) {
if(local.get()==null){
local.set(new Integer(1));
}
int num=local.get().intValue();
num=num+1;
local.set(new Integer(num));
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + local.get().intValue());
}
}
}

public class TestMan {
public static void main(String[] args) {
Runnable safe=new ThreadSafe();
Thread thread1=new Thread(safe,"thread1");
Thread thread2=new Thread(safe,"thread2");
thread1.start();
thread2.start();
}
}

 運(yùn)行結(jié)果

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

結(jié)果是一樣的，所以這里變量安全有３個(gè)辦法可以解決。

然后在說(shuō)說(shuō)線程的同步的問(wèn)題。我們看上面的運(yùn)行結(jié)果。

num is value +===thread2---------2
num is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread2---------3
num is value +===thread1---------4
num is value +===thread2---------4

就　可以看出他們不是線程同步的，是thread1和thread2在交替執(zhí)行的。在有些情況下，要求一段代碼在運(yùn)行的過(guò)程中是一個(gè)不可分割的實(shí) 體，就是原子的。就是說(shuō)當(dāng)已經(jīng)有線程在執(zhí)行這段代碼的時(shí)候，其他的線程必須等待他執(zhí)行完畢后才能竟來(lái)執(zhí)行，這就是所謂的線程同步。

java通過(guò)同步鎖來(lái)執(zhí)行線程的同步和等待，也就是說(shuō)，要不間斷執(zhí)行的代碼需要放在synchronized關(guān)鍵字標(biāo)識(shí)的代碼塊中。可以用來(lái)修飾代 碼塊，也可以修飾方法。

public class ThreadSafe implements java.lang.Runnable{
public synchronized void run() {
int num = 1;
for (int i = 0; i < 3; i++) {
num = num + 1;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("num is value +==="+Thread.currentThread().getName()+"---------" + num);
}
}
}

public class TestMan {
public static void main(String[] args) {
Runnable safe=new ThreadSafe();
Thread thread1=new Thread(safe,"thread1");
Thread thread2=new Thread(safe,"thread2");
thread1.start();
thread2.start();
}
}

 運(yùn)行結(jié)果

um is value +===thread1---------2
num is value +===thread1---------3
num is value +===thread1---------4
num is value +===thread2---------2
num is value +===thread2---------3
num is value +===thread2---------4

可以看到thread1運(yùn)行結(jié)束后thread2才開(kāi)始運(yùn)行的。代碼還可以這么寫(xiě)

public class ThreadSafe implements java.lang.Runnable {
public void run() {
int num = 1;
synchronized (this) {
for (int i = 0; i < 3; i++) {
num = num + 1;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("num is value +==="
+ Thread.currentThread().getName() + "---------" + num);
}
}
}
}

在啟用同步鎖機(jī)制以后，需要避免

１無(wú)線等待，，線程Ｂ等待線程Ａ執(zhí)行完畢，然后線程Ａ確進(jìn)入了死循環(huán)。