java多线程2：局部变量的线程安全，实列变量的非线程安全

java多线程2：局部变量的线程安全，实列变量的非线程安全

“非线程安全“就是在多个线程访问同一个对象的实例变量进行并发访问时候发生，产生的后果就是”脏读“，也就是取到的数据其实是被修改过的。

a.多线程访问局部变量是线程安全的。

package multiThread.synchro;

public class OneNumber {
	

	public void receiveUserName(String userName) {
		int num = 0;
		if (userName.equals("a")) {
			num = 100;
		} else {
			num = 200;
		}
		System.out.println("userName：" + userName + "     数字是：" + num);
	}
}

package multiThread.synchro;

public class AThread extends Thread {
	private OneNumber oneNumber;

	public AThread(OneNumber oneNumber) {
		this.oneNumber = oneNumber;
	}

	public void run() {
		oneNumber.receiveUserName("a");
	}
}

package multiThread.synchro;

public class BThread extends Thread {
	private OneNumber oneNumber;

	public BThread(OneNumber oneNumber) {
		this.oneNumber = oneNumber;
	}

	public void run() {
		oneNumber.receiveUserName("b");
	}
}

package multiThread.synchro;

public class Main {
	public static void main(String[] args) {
		OneNumber oneNumber = new OneNumber();
		AThread aThread = new AThread(oneNumber);
		BThread bThread = new BThread(oneNumber);
		aThread.start();
		
		bThread.start();
	}
}

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b.如果对上边OneNumber类稍作修改，将局部变量num变为实例变量：

package multiThread.synchro;

public class OneNumber {

	int num = 0;

	public void receiveUserName(String userName) {

		if (userName.equals("a")) {
			num = 100;
		} else {
			num = 200;
		}
		System.out.println("userName：" + userName + "     数字是：" + num);
	}
}

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c.如何解决并发造成的访问实例变量的非线程安全问题：

加关键字：synchronized

package multiThread.synchro;

public class OneNumber {

	int num = 0;

	synchronized public void receiveUserName(String userName) {

		if (userName.equals("a")) {
			num = 100;
		} else {
			num = 200;
		}
		System.out.println("userName：" + userName + "     数字是：" + num);
	}
}

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