《Java多线程编程核心技术》读后感(十二)
2024-08-28 17:13:54
类ThreadLocal的使用
主要解决的是每个线程绑定自己的值,可以将ThreadLocal类比喻成全局存放数据的盒子,盒子中可以存储每个线程私有数据。
类ThreadLocal解决的是变量在不同线程间的隔离线,也就是不同线程拥有自己的值,不同线程中的值是可以放入ThreadLocal类中进行保存的
方法get()与null
package Third; public class Run {
public static ThreadLocal tl = new ThreadLocal(); public static void main(String[] args) {
if (tl.get() == null) {
System.out.println("从未放过值");
tl.set("我的值");
}
System.out.println(tl.get());
System.out.println(tl.get());
} }
验证线程变量的隔离性
package Third; public class ThreadA extends Thread { @Override
public void run() {
try {
for (int i = 0; i < 100; i++) {
if (Tools.tl.get() == null) {
Tools.tl.set("ThreadA" + (i + 1));
} else {
System.out.println("ThreadA get Value=" + Tools.tl.get());
}
Thread.sleep(200);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
} }
package Third; public class ThreadB extends Thread { @Override
public void run() {
try {
for (int i = 0; i < 100; i++) {
if (Tools.tl.get() == null) {
Tools.tl.set("ThreadB" + (i + 1));
} else {
System.out.println("ThreadB get Value=" + Tools.tl.get());
}
Thread.sleep(200);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
} }
package Third; public class Tools { public static ThreadLocal tl = new ThreadLocal(); }
package Third; public class Run { public static void main(String[] args) { try {
ThreadA a = new ThreadA();
ThreadB b = new ThreadB();
a.start();
b.start(); for (int i = 0; i < 100; i++) {
if (Tools.tl.get() == null) {
Tools.tl.set("Main" + (i + 1));
} else {
System.out.println("Main get Value=" + Tools.tl.get());
}
Thread.sleep(200);
}
} catch (InterruptedException e) {
e.printStackTrace();
} } }
每个线程取出自己的数据
package Third; import java.util.Date; public class ThreadA extends Thread { @Override
public void run() {
try {
for (int i = 0; i < 20; i++) {
if (Tools.tl.get() == null) {
Tools.tl.set(new Date());
}
System.out.println("A " + Tools.tl.get().getTime());
Thread.sleep(100);
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
package Third; import java.util.Date; public class ThreadB extends Thread { @Override
public void run() {
try {
for (int i = 0; i < 20; i++) {
if (Tools.tl.get() == null) {
Tools.tl.set(new Date());
}
System.out.println("B " + Tools.tl.get().getTime());
Thread.sleep(100);
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
package Third; import java.util.Date; public class Tools { public static ThreadLocal<Date> tl = new ThreadLocal<Date>(); }
package Third; public class Run { public static void main(String[] args) {
try {
ThreadA a = new ThreadA();
a.start(); Thread.sleep(1000); ThreadB b = new ThreadB();
b.start();
} catch (InterruptedException e) {
e.printStackTrace();
} } }
怎么实现第一次调用get()不返回null呢?也就是具有默认值的效果
解决get()返回null问题
package Third; public class ThreadLocalExt extends ThreadLocal {
@Override
protected Object initialValue() {
return "我是默认值 第一次get不再为null";
}
}
package Third; public class Run {
public static ThreadLocalExt tl = new ThreadLocalExt(); public static void main(String[] args) {
if (tl.get() == null) {
System.out.println("从未放过值");
tl.set("我的值");
}
System.out.println(tl.get());
System.out.println(tl.get());
} }
此案例仅仅证明main线程有自己的值,那其他线程呢?
再次验证线程变量的隔离性
package Third; public class Tools {
public static ThreadLocalExt tl = new ThreadLocalExt();
}
package Third; import java.util.Date; public class ThreadLocalExt extends ThreadLocal {
@Override
protected Object initialValue() {
return new Date().getTime();
}
}
package Third; public class ThreadA extends Thread { @Override
public void run() {
try {
for (int i = 0; i < 10; i++) {
System.out.println("在ThreadA线程中取值=" + Tools.tl.get());
Thread.sleep(100);
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
package Third; public class Run { public static void main(String[] args) {
try {
for (int i = 0; i < 10; i++) {
System.out.println(" 在Main线程中取值=" + Tools.tl.get());
Thread.sleep(100);
}
Thread.sleep(5000);
ThreadA a = new ThreadA();
a.start();
} catch (InterruptedException e) {
e.printStackTrace();
}
} }
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