ClassLoader

ClassLodingLinkingInitializing.md

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+## 3：类加载-初始化
+
+1. 加载过程
+   1. Loading
+
+      1. 双亲委派，主要出于安全来考虑
+
+      2. LazyLoading 五种情况
+
+         1. –new getstatic putstatic invokestatic指令，访问final变量除外
+
+            –java.lang.reflect对类进行反射调用时
+
+            –初始化子类的时候，父类首先初始化
+
+            –虚拟机启动时，被执行的主类必须初始化
+
+            –动态语言支持java.lang.invoke.MethodHandle解析的结果为REF_getstatic REF_putstatic REF_invokestatic的方法句柄时，该类必须初始化
+
+      3. ClassLoader的源码
+
+         1. findInCache -> parent.loadClass -> findClass()
+
+      4. 自定义类加载器
+
+         1. extends ClassLoader
+         2. overwrite findClass() -> defineClass(byte[] -> Class clazz)
+         3. 加密
+
+      5. 混合执行 编译执行 解释执行
+
+         1. 检测热点代码：-XX:CompileThreshold = 10000
+   2. Linking 
+      1. Verification
+      2. Preparation
+      3. Resolution
+   3. Initializing

src/com/mashibing/jvm/Hello.java

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+package com.mashibing.jvm;
+
+public class Hello {
+    public void m() {
+        System.out.println("Hello JVM!");
+    }
+}

src/com/mashibing/jvm/classloader/T007_MSBClassLoaderWithEncription.java

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+package com.mashibing.jvm.classloader;
+
+import com.mashibing.jvm.Hello;
+
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+
+public class T007_MSBClassLoaderWithEncription extends ClassLoader {
+
+    public static int seed = 0B10110110;
+
+    @Override
+    protected Class<?> findClass(String name) throws ClassNotFoundException {
+        File f = new File("c:/test/", name.replace('.', '/').concat(".msbclass"));
+
+        try {
+            FileInputStream fis = new FileInputStream(f);
+            ByteArrayOutputStream baos = new ByteArrayOutputStream();
+            int b = 0;
+
+            while ((b=fis.read()) !=0) {
+                baos.write(b ^ seed);
+            }
+
+            byte[] bytes = baos.toByteArray();
+            baos.close();
+            fis.close();//可以写的更加严谨
+
+            return defineClass(name, bytes, 0, bytes.length);
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+        return super.findClass(name); //throws ClassNotFoundException
+    }
+
+    public static void main(String[] args) throws Exception {
+
+        encFile("com.mashibing.jvm.hello");
+
+        ClassLoader l = new T007_MSBClassLoaderWithEncription();
+        Class clazz = l.loadClass("com.mashibing.jvm.Hello");
+        Hello h = (Hello)clazz.newInstance();
+        h.m();
+
+        System.out.println(l.getClass().getClassLoader());
+        System.out.println(l.getParent());
+    }
+
+    private static void encFile(String name) throws Exception {
+        File f = new File("c:/test/", name.replace('.', '/').concat(".class"));
+        FileInputStream fis = new FileInputStream(f);
+        FileOutputStream fos = new FileOutputStream(new File("c:/test/", name.replaceAll(".", "/").concat(".msbclass")));
+        int b = 0;
+
+        while((b = fis.read()) != -1) {
+            fos.write(b ^ seed);
+        }
+
+        fis.close();
+        fos.close();
+    }
+}

src/com/mashibing/jvm/classloader/T008_LazyLoading.java

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+package com.mashibing.jvm.classloader;
+
+public class T008_LazyLoading { //严格讲应该叫lazy initialzing，因为java虚拟机规范并没有严格规定什么时候必须loading,但严格规定了什么时候initialzing
+    public static void main(String[] args) throws Exception {
+        //P p;
+        //X x = new X();
+        //System.out.println(P.i);
+        //System.out.println(P.j);
+        Class.forName("com.mashibing.jvm.classloader.T008_LazyLoading$P");
+
+    }
+
+    public static class P {
+        final static int i = 8;
+        static int j = 9;
+        static {
+            System.out.println("P");
+        }
+    }
+
+    public static class X extends P {
+        static {
+            System.out.println("X");
+        }
+    }
+}

src/com/mashibing/jvm/classloader/T009_WayToRun.java

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+package com.mashibing.jvm.classloader;
+
+public class T009_WayToRun {
+    public static void main(String[] args) {
+        for(int i=0; i<10_0000; i++)
+            m();
+
+        long start = System.currentTimeMillis();
+        for(int i=0; i<10_0000; i++) {
+            m();
+        }
+        long end = System.currentTimeMillis();
+        System.out.println(end - start);
+    }
+
+    public static void m() {
+        for(long i=0; i<10_0000L; i++) {
+            long j = i%3;
+        }
+    }
+}

src/com/mashibing/jvm/cms/Student.java

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+package com.mashibing.jvm.cms;
+
+public class Student {
+    private String name;
+    private int age;
+
+    public Student(String name, int age) {
+        this.name = name;
+        this.age = age;
+    }
+}

src/com/mashibing/jvm/cms/Test.java

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+package com.mashibing.jvm.cms;
+
+
+import java.util.ArrayList;
+
+public class Test {
+
+    public static void main(String[] args) {
+
+
+        ArrayList<Student> list = new ArrayList<>();
+        for(int i = 0;i<6000000;i++){
+            list.add(new Student("msb"+i,i));
+        }
+
+        try {
+            Thread.sleep(10000000000L);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+}

src/com/mashibing/jvm/gc/T15_FullGC_Problem01.java

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+package com.mashibing.jvm.gc;
+
+import java.math.BigDecimal;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * 从数据库中读取信用数据，套用模型，并把结果进行记录和传输
+ */
+
+public class T15_FullGC_Problem01 {
+
+    private static class CardInfo {
+        BigDecimal price = new BigDecimal(0.0);
+        String name = "张三";
+        int age = 5;
+        Date birthdate = new Date();
+
+        public void m() {
+
+        }
+    }
+
+    private static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(50,
+            new ThreadPoolExecutor.DiscardOldestPolicy());
+
+    public static void main(String[] args) throws Exception {
+        executor.setMaximumPoolSize(50);
+
+        for (;;){
+            modelFit();
+            Thread.sleep(100);
+        }
+    }
+
+    private static void modelFit(){
+        List<CardInfo> taskList = getAllCardInfo();
+        taskList.forEach(info -> {
+            // do something
+            executor.scheduleWithFixedDelay(() -> {
+                //do sth with info
+                info.m();
+
+            }, 2, 3, TimeUnit.SECONDS);
+        });
+    }
+
+    private static List<CardInfo> getAllCardInfo(){
+        List<CardInfo> taskList = new ArrayList<>();
+
+        for (int i = 0; i < 100; i++) {
+            CardInfo ci = new CardInfo();
+            taskList.add(ci);
+        }
+
+        return taskList;
+    }
+}