add blog

noakcn · Jun 11, 2018 · be69e5e · be69e5e
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diff --git a/source/_posts/20180611多次start线程.md b/source/_posts/20180611多次start线程.md
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+---
+title: 多次start一个线程会怎么样
+date: 2018-06-11 15:20:22
+tags:
+- java
+- 多线程
+categories:
+- 笔记
+---
+
+我们都知道，java中通过多线程来提高cpu的利用率，并且提供了三种实现方式
+- 继承Thread类，重写run方法
+- 实现Runnable接口，重写run方法，利用Thread类来启动
+- 通过Callable和FutureTask创建线程
+
+本次例子使用Thread类和Runnable来探讨我们对同一个线程**多次调用start()**会发生什么。
+
+# 代码
+
+线程例子如下：
+```
+public class ThreadDemo implements Runnable {
+
+    @Override
+    public void run() {
+        while (true) {
+            System.out.println(Thread.currentThread().getName() + " 线程启动run");
+            try {
+                Thread.sleep(1000);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+}
+```
+
+测试类
+```
+/**
+ * 多次调用线程start
+ */
+public class RepeatedlyRunStart {
+
+    @Test
+    public void test() {
+        Thread thread = new Thread(new ThreadDemo());
+        thread.start();
+        thread.start();
+    }
+}
+```
+
+# 结果
+运行结果如下：
+```
+Thread-0 线程启动run
+
+java.lang.IllegalThreadStateException
+	at java.lang.Thread.start(Unknown Source)
+	at com.noak.myblogexample.part7.RepeatedlyRunStart.test(RepeatedlyRunStart.java:14)
+	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
+	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
+	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
+	at java.lang.reflect.Method.invoke(Unknown Source)
+	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
+	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
+	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
+	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
+	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
+	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
+	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
+	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
+	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
+	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
+	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
+	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
+	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
+	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
+	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
+	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
+	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
+	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
+```
+
+# 分析
+从运行结果来看，我们第一次的start是成功运行了线程，第二次调用start我们收到了一个异常`IllegalThreadStateException` 测试线程被结束了。
+
+`IllegalThreadStateException`异常的意思是线程状态异常。这是因为java中线程是有**生命周期（状态）**的。单我们创建了线程，并执行start后，线程状态就进入了就绪状态，不再是新建状态。
+
+不过线程的run方法是可以被反复调用的。