線程安全的本質體現在兩個方面，

　　Ａ變量安全：多線程同時運行一段代碼

　　Ｂ線程同步：一個線程還沒執行完，另一個線程又進來接著執行。

　看個簡單的例子。

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);
}
}
}

 運行結果

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

很明顯是錯誤的，應為兩個線程共享同一個變量。這里就是變量的安全問題。

解決辦法：

１拋棄單實例，多線程的方式，用多實例，多線程的方式，這樣就和單線程是一個樣了，不會出錯，但是是最接近傳統的編程模式

２不要用類的實例變量，經可能把變量封裝到方法內部。

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();
}
}

運行結果

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();
}
}

 運行結果

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辦法，就是把變量經可能的封裝到風發內部，這樣他們就是線程的私有變量了。另外，從jdk1.2后，推出了 threadlocal　對象，它作為線程的一個局部變量，可以為每個線程創建一個副本，用來保存每個線程的屬性，各是各的，互不干擾。單每個 threadlocal變量只能保存一個變量，假如有多個變量要保存，那么就要寫多個threadlocal對象。

我們把代碼改寫一下。

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();
}
}

 運行結果

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

結果是一樣的，所以這里變量安全有３個辦法可以解決。

然后在說說線程的同步的問題。我們看上面的運行結果。

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在交替執行的。在有些情況下，要求一段代碼在運行的過程中是一個不可分割的實 體，就是原子的。就是說當已經有線程在執行這段代碼的時候，其他的線程必須等待他執行完畢后才能竟來執行，這就是所謂的線程同步。

java通過同步鎖來執行線程的同步和等待，也就是說，要不間斷執行的代碼需要放在synchronized關鍵字標識的代碼塊中。可以用來修飾代 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();
}
}

 運行結果

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運行結束后thread2才開始運行的。代碼還可以這么寫

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);
}
}
}
}

在啟用同步鎖機制以后，需要避免

１無線等待，，線程Ｂ等待線程Ａ執行完畢，然后線程Ａ確進入了死循環。