[add] add class loading example in jvm module

brianway · brianway · commit 6840e59180a4 · 2017-03-19T22:54:58.000+08:00
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/ConstClass.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/ConstClass.java
@@ -0,0 +1,15 @@
+package com.brianway.learning.java.jvm.classfile;
+
+/**
+ * Created by brian on 17/3/19.
+ * 被动使用类字段演示
+ * 常量在编译阶段会存入调用类的常量池,本质上没有引用调用常量的类,
+ * 不会触发定义常量的类的初始化
+ */
+public class ConstClass {
+    static {
+        System.out.println("ConsClass init!");
+    }
+
+    public static final String HELLOWORLD = "hello world";
+}
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/NoInitialization.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/NoInitialization.java
@@ -0,0 +1,21 @@
+package com.brianway.learning.java.jvm.classfile;
+
+/**
+ * Created by brian on 17/3/19.
+ * 非主动使用类字段演示
+ * 1. 静态字段只有直接定义这个字段的类才会被初始化
+ * 2. 通过数组来定义引用类,不会触发类的初始化
+ * (-XX:+TraceClassLoading,对于 HotSpot 虚拟机可观察到子类的加载)
+ */
+public class NoInitialization {
+    public static void main(String[] args) {
+        System.out.println(SubClass.value);
+        SubClass[] sca = new SubClass[10];
+
+        System.out.println(ConstClass.HELLOWORLD);
+    }
+}
+/*
+SuperClass init!
+123
+ */
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SimpleClass.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SimpleClass.java
@@ -0,0 +1,14 @@
+package com.brianway.learning.java.jvm.classfile;
+
+/**
+ * Created by brian on 17/3/15.
+ * 一段简单的 Java 代码,用于分析字节码
+ * javap -verbose SimpleClass.java
+ */
+public class SimpleClass {
+    private int m;
+
+    public int inc() {
+        return m + 1;
+    }
+}
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SubClass.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SubClass.java
@@ -0,0 +1,10 @@
+package com.brianway.learning.java.jvm.classfile;
+
+/**
+ * Created by brian on 17/3/19.
+ */
+public class SubClass extends SuperClass {
+    static {
+        System.out.println("SubClass init!");
+    }
+}
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SuperClass.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/classfile/SuperClass.java
@@ -0,0 +1,14 @@
+package com.brianway.learning.java.jvm.classfile;
+
+/**
+ * Created by brian on 17/3/19.
+ * 被动引用的例子
+ * 通过子类引用父类的静态字段,不会导致子类初始化
+ */
+public class SuperClass {
+    static {
+        System.out.println("SuperClass init!");
+    }
+
+    public static int value = 123;
+}
