first commit

.gitignore

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+### IntelliJ IDEA ###
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_21" default="true" project-jdk-name="21" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/java_test.iml" filepath="$PROJECT_DIR$/java_test.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

java_test.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

src/Main.java

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+public class Main {
+    public static void main(String[] args) {
+        System.out.println("Hello, World!");
+    }
+}

src/MultiThreadDemo.java

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+import java.lang.management.ThreadInfo;
+import java.lang.management.ThreadMXBean;
+
+public class MultiThreadDemo {
+
+    public static void main(String[] args) {
+        ThreadMXBean threadMXBean = java.lang.management.ManagementFactory.getThreadMXBean();
+
+        // 不需要获取同步的 monitor 和 synchronizer 信息，仅获取线程和线程堆栈信息
+        ThreadInfo[] threadInfos = threadMXBean.dumpAllThreads(false, false);
+        // 遍历线程信息，仅打印线程 ID 和线程名称信息
+        for (ThreadInfo threadInfo : threadInfos) {
+
+            System.out.println("[" + threadInfo.getThreadId() + "] "
+                    + threadInfo.getThreadName()
+                    + "; Thread state: " + threadInfo.getThreadState()
+                    + " ; Is daemon: " + threadInfo.isDaemon()
+            );
+        }
+    }
+}

src/ProcessDemo.java

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+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+
+public class ProcessDemo {
+
+    public static void main(String[] args) {
+        try {
+            Process p = Runtime.getRuntime().exec(new String[]{"ls","-l", "/home/admin"});
+
+            // 读取标准输出
+            BufferedReader stdInput = new BufferedReader(new InputStreamReader(p.getInputStream()));
+            String line;
+            while ((line = stdInput.readLine()) != null) {
+                System.out.println("标准输出: " + line);
+            }
+
+            // 读取错误输出
+            BufferedReader stdError = new BufferedReader(new InputStreamReader(p.getErrorStream()));
+            while ((line = stdError.readLine()) != null) {
+                System.out.println("错误输出: " + line);
+            }
+
+            // 等待进程结束
+            int exitCode = p.waitFor();
+
+            System.out.println("进程结束，退出码: " + exitCode);
+        } catch (IOException | InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

src/ThreadLocalDemo.java

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+public class ThreadLocalDemo {
+
+    public ThreadLocal<Integer> localVar = new ThreadLocal<>();
+    private Integer integer = 0;
+
+    public void add() {
+        integer = integer + 1;
+    }
+
+    public int get() {
+        return  integer;
+    }
+
+    public static void main(String[] args) {
+        ThreadLocalDemo threadLocalDemo = new ThreadLocalDemo();
+
+        threadLocalDemo.localVar.set(threadLocalDemo.integer);
+
+        Thread t1 = new Thread(threadLocalDemo::add, "t1");
+
+        Thread t2 = new Thread( () -> {
+            threadLocalDemo.add();
+            threadLocalDemo.localVar.set(threadLocalDemo.get());
+            System.out.println("value in new thread: " + threadLocalDemo.get());
+            System.out.println("value in new thread localVar: " + threadLocalDemo.localVar.get());
+        }, "t2");
+
+        t1.start();
+        t2.start();
+
+        try {
+            t1.join();
+            System.out.println("value in main thread: " + threadLocalDemo.get());
+            System.out.println("value in main thread localVar: " + threadLocalDemo.localVar.get());
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+
+
+}

src/ThreadShareVariable.java

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+import java.util.stream.DoubleStream;
+
+/**
+ * 类ThreadShareVariable演示了线程间共享变量的使用和潜在风险
+ */
+public class ThreadShareVariable {
+
+    /**
+     * 共享变量i，多个线程会访问和修改这个变量
+     */
+    public Integer i = 0;
+
+    /**
+     * 设置共享变量i的值
+     *
+     * @param i 新的变量值
+     */
+    public void setI(int i) {
+        this.i = i;
+    }
+
+    /**
+     * 获取共享变量i的值
+     *
+     * @return 当前的变量值
+     */
+    public int getI() {
+        return i;
+    }
+
+    /**
+     * 主函数演示了如何在多线程环境下使用共享变量，并展示了线程局部变量的用法
+     *
+     * @param args 命令行参数
+     */
+    public static void main(String[] args) {
+        ThreadShareVariable threadShareVariable = new ThreadShareVariable();
+
+        // 使用ThreadLocal以线程为单位隔离变量i的值，避免共享变量的并发问题
+        ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(threadShareVariable::getI);
+
+        // 新建一个线程，该线程会修改共享变量i的值
+        Thread thread = new Thread(() -> {
+            threadShareVariable.setI(1);
+        });
+
+        // 新建一个线程，该线程会打印共享变量i的值
+        new Thread(() -> {
+            System.out.println("print i in new thread: " + threadShareVariable.getI());
+
+        }).start();
+
+        thread.start();
+        try {
+            // 等待线程thread执行完毕
+            thread.join();
+            // 打印共享变量i的值，预期为1
+            System.out.println(threadShareVariable.getI());
+            // 打印ThreadLocal中的值，此处的值是最原始的共享变量值0
+            System.out.println("value in ThreadLocal: " + threadLocal.get());
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+
+    }
+}

src/WeakReferenceExample.java

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+import java.lang.ref.WeakReference;
+
+public class WeakReferenceExample {
+
+    public static void main(String[] args) {
+        Object obj = new Object();
+
+        //创建一个弱引用，指向该对象
+        WeakReference<Object> weakRef = new WeakReference<>(obj);
+
+        //手动调用垃圾回收
+        System.gc();
+
+        //检查弱引用是否被回收
+        if (weakRef.get() == null) {
+            System.out.println("Weak reference has been collected.");
+        } else {
+            System.out.println("Weak reference still exists.");
+        }
+
+        //清除强引用
+        obj = null;
+
+        if (weakRef.get() == null) {
+            System.out.println("Weak reference has been collected.");
+        } else {
+            System.out.println("Weak reference still exists.");
+        }
+
+        //再次手动调用垃圾回收
+        System.gc();
+
+        //再次检查弱引用是否被回收
+        if (weakRef.get() == null) {
+            System.out.println("Weak reference has been collected.");
+        } else {
+            System.out.println("Weak reference still exists.");
+        }
+    }
+}