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各种学习
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‎src/main/java/JVM/MethodAreaTest.java‎

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package JVM;
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import typeInfo.compilerAPI.JavaCompilerTask;
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/**
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* 方法区测试
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* jdk1.7及以前 永久区（Perm）,-XX:PermSize ,-XX:MaxPermSize
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* jdk1.8及以后，元数据区，-XX:MetaspaceSize,-XX:MaxMetaspaceSize
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*
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* @author ：chenxin
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* @date ：Created in 2020/3/26 13:36
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* @description：方法区测试
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* @modified By：
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* @version: $
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*/
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public class MethodAreaTest {
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public static void main(String[] args) {
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new MethodAreaTest().loadClass();
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}
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void loadClass() {
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String className = "Hello";
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for (int i = 0; i < 10000; i++) {
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String realName = className + i;
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System.out.println(i);
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StringBuilder s = new StringBuilder();
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s.append("public class " + realName + "{\n");
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s.append("public static void main(String[] args){\n");
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s.append("System.out.println(\"Hello World\");\n");
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s.append("}\n");
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s.append("}");
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Class<?> compile = JavaCompilerTask.compile(realName, s.toString());
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}
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}
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}

‎src/main/java/JVM/gc/HeapAlloc.java‎

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package JVM.gc;
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/**
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* 来自于《实战Java虚拟机》
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* -Xms5M -Xmx20M -XX:+PrintGCDetails -XX:+PrintCommandLineFlags -XX:+UseSerialGC
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* @author Tom
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*
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*/
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public class HeapAlloc {
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public static void main(String[] args) {
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printMemoryInfo();
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byte[] b = new byte[1*1024*1024];
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System.out.println("分配1MB空间");
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printMemoryInfo();
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b = new byte[4*1024*1024];
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System.out.println("分配4MB空间");
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printMemoryInfo();
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}
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public static void printMemoryInfo()
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{
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System.out.print("maxMemory=");
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System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024.0 + " MB");
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System.out.print("freeMemory=");
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System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024.0 + " MB");
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System.out.print("totalMemory=");
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System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024.0 + " MB");
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}
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}

‎src/main/java/JVM/gc/NewSizeDemo.java‎

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package JVM.gc;
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/**
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* 来自于《实战Java虚拟机》
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* -Xmx20m -Xms20m -Xmn1m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC
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* 新生代1M，eden/s0=2, eden 512KB, s0=s1=256KB
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* 新生代无法容纳1M，所以直接放老年代
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*
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* -Xmx20m -Xms20m -Xmn7m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC
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* 新生代7M，eden/s0=2, eden=3.5M, s0=s1=1.75M
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* 所以可以容纳几个数组，但是无法容纳所有，因此发生GC
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*
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* -Xmx20m -Xms20m -Xmn15m -XX:SurvivorRatio=8 -XX:+PrintGCDetails -XX:+UseSerialGC
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* 新生代15M，eden/s0=8, eden=12M, s0=s1=1.5M
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*
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* -Xmx20m -Xms20m -XX:NewRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC
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* 新生代是6.6M，老年代13.3M
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* @author Tom
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*
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*/
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public class NewSizeDemo {
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public static void main(String[] args) {
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byte[] b = null;
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for(int i=0;i<10;i++)
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{
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b = new byte[1*1024*1024];
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}
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}
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}

‎src/main/java/JVM/gc/reference/PhantomReferenceTest.java‎

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package JVM.gc.reference;
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import java.lang.ref.PhantomReference;
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import java.lang.ref.ReferenceQueue;
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/**
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* 虚引用测试
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*
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* @author ：chenxin
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* @date ：Created in 2020/3/26 15:01
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* @description：虚引用测试
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* @modified By：
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* @version: $
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*/
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public class PhantomReferenceTest {
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public static void main(String[] args) {
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ReferenceQueue<StringBuilder> queue = new ReferenceQueue<StringBuilder>();
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// final CheckReferenceQueue checkReferenceQueue = new CheckReferenceQueue(queue);
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StringBuilder s1 = new StringBuilder();
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for (int i = 0; i < 10000; i++) {
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s1.append("00000000000");
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}
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// PhantomReference<StringBuilder> s2 = new PhantomReference<StringBuilder>(s1);
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s1 = null;
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}
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}

‎src/main/java/JVM/gc/reference/SoftReferenceTest.java‎

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package JVM.gc.reference;
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import java.lang.ref.SoftReference;
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/**
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* 软引用测试
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*
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* @author ：chenxin
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* @date ：Created in 2020/3/26 14:38
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* @description：软引用测试
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* @modified By：
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* @version: $
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*/
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public class SoftReferenceTest {
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public static void main(String[] args) {
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StringBuilder s1 = new StringBuilder();
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for (int i = 0; i < 10000; i++) {
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s1.append("0000");
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}
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SoftReference<StringBuilder> s2 = new SoftReference<StringBuilder>(s1);
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s1 = null;
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System.out.println(s2.get().length());
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System.gc();
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//软引用，内存不紧张，则不会回收
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System.out.println(s2.get().length());
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final byte[] bytes = new byte[(int) (1024 * 1024 * 3)];
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System.gc();
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System.out.println(s2.get() == null);
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}
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}

‎src/main/java/JVM/gc/reference/StrongReferenceTest.java‎

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package JVM.gc.reference;
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/**
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* 强引用测试
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*
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* @author ：chenxin
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* @date ：Created in 2020/3/26 14:32
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* @description：强引用测试
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* @modified By：
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* @version: $
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*/
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public class StrongReferenceTest {
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public static void main(String[] args) {
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StringBuilder s1 = new StringBuilder();
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for (int i = 0; i < 10000; i++) {
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s1.append("00000000000000000000");
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}
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StringBuilder s2 = s1;
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//s1为null但是s2依然占据着内存
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s1 = null;
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// s2 = null;
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System.gc();
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//垃圾回收，当时不能回收强类型的引用，内存被占用 ，下面语句报错
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final byte[] bytes = new byte[1024 * 1024 * 3];
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}
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}

‎src/main/java/JVM/gc/reference/WeakReferenceTest.java‎

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package JVM.gc.reference;
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import java.lang.ref.SoftReference;
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import java.lang.ref.WeakReference;
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/**
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* 弱引用测试
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*
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* @author ：chenxin
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* @date ：Created in 2020/3/26 14:51
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* @description：弱引用测试
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* @modified By：
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* @version: $
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*/
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public class WeakReferenceTest {
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public static void main(String[] args) {
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StringBuilder s1 = new StringBuilder();
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for (int i = 0; i < 10000; i++) {
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s1.append("00000000000");
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}
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WeakReference<StringBuilder> s2 = new WeakReference<StringBuilder>(s1);
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s1 = null;
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System.out.println(s2.get().length());
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//弱引用只要gc就会被回收
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System.gc();
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System.out.println(s2 == null);
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Byte[] bytes = new Byte[(int) (1024 * 1024 * 3.5)];
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}
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}

‎src/main/java/JVM/jmx/MBean/Hello.java‎

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package JVM.jmx.MBean;
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/* Hello.java - MBean implementation for the Hello World MBean.
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This class must implement all the Java methods declared in the
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HelloMBean interface, with the appropriate behavior for each one. */
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public class Hello implements HelloMBean {
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public void sayHello() {
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System.out.println("hello, world");
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}
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public int add(int x, int y) {
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return x + y;
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}
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/*
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* Getter for the Name attribute. The pattern shown here is frequent: the getter
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* returns a private field representing the attribute value. In our case, the
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* attribute value never changes, but for other attributes it might change as
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* the application runs. Consider an attribute representing statistics such as
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* uptime or memory usage, for example. Being read-only just means that it can't
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* be changed through the management interface.
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*/
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public String getName() {
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return this.name;
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}
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/*
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* Getter for the CacheSize attribute. The pattern shown here is frequent: the
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* getter returns a private field representing the attribute value, and the
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* setter changes that field.
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*/
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public int getCacheSize() {
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return this.cacheSize;
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}
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/*
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* Setter for the CacheSize attribute. To avoid problems with stale values in
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* multithreaded situations, it is a good idea for setters to be synchronized.
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*/
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public synchronized void setCacheSize(int size) {
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this.cacheSize = size;
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/*
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* In a real application, changing the attribute would typically have effects
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* beyond just modifying the cacheSize field. For example, resizing the cache
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* might mean discarding entries or allocating new ones. The logic for these
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* effects would be here.
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*/
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System.out.println("Cache size now " + this.cacheSize);
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}
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private final String name = "Reginald";
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private int cacheSize = DEFAULT_CACHE_SIZE;
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private static final int DEFAULT_CACHE_SIZE = 200;
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}

‎src/main/java/JVM/jmx/MBean/HelloMBean.java‎

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package JVM.jmx.MBean;
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public interface HelloMBean {
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// operations
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public void sayHello();
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public int add(int x, int y);
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// attributes
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// a read-only attribute called Name of type String
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public String getName();
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// a read-write attribute called CacheSize of type int
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public int getCacheSize();
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public void setCacheSize(int size);
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}

‎src/main/java/JVM/jmx/MBean/Main.java‎

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package JVM.jmx.MBean;
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/* Main.java - main class for Hello World example. Create the
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HelloWorld MBean, register it, then wait forever (or until the
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program is interrupted). */
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import java.lang.management.*;
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import javax.management.*;
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public class Main {
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/*
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* For simplicity, we declare "throws Exception". Real programs will usually
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* want finer-grained exception handling.
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*/
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public static void main(String[] args) throws Exception {
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// Get the Platform MBean Server
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MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
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// Construct the ObjectName for the MBean we will register
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ObjectName name = new ObjectName("com.example.mbeans:type=Hello");
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// Create the Hello World MBean
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Hello mbean = new Hello();
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// Register the Hello World MBean
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mbs.registerMBean(mbean, name);
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// Wait forever
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System.out.println("Waiting forever...");
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Thread.sleep(Long.MAX_VALUE);
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}
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}

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