Integer的缓存池

JDK7源码学习.iml

@@ -7,6 +7,5 @@
     </content>
     <orderEntry type="inheritedJdk" />
     <orderEntry type="sourceFolder" forTests="false" />
-    <orderEntry type="library" name="lib" level="project" />
   </component>
 </module>

jdkTest/jdkTest.iml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <module type="JAVA_MODULE" version="4">
-  <component name="NewModuleRootManager" inherit-compiler-output="true">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_7" inherit-compiler-output="true">
     <exclude-output />
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />

jdkTest/src/com/jdk/test/TestHashMap.java

@@ -0,0 +1,23 @@
+package com.jdk.test;
+
+import org.junit.Test;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+
+/**
+ * @author ÍõƼ
+ * @date 2017/12/8 0008
+ */
+public class TestHashMap {
+
+    @Test
+    public void indexForBucket() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
+        HashMap<String, String> map = new HashMap<>();
+        Method hash = map.getClass().getDeclaredMethod("hash", Object.class);
+        hash.setAccessible(true);
+        int hashOfWanwanpp = (int)hash.invoke(map, "wanwanpp");
+        System.out.println(Integer.toBinaryString(hashOfWanwanpp));
+    }
+}

jdkTest/src/com/jdk/test/TestUnsafe.java

@@ -1,5 +1,6 @@
 package com.jdk.test;
 
+import org.junit.Test;
 import sun.misc.Unsafe;
 
 import java.lang.reflect.Field;
@@ -24,7 +25,7 @@ public class TestUnsafe {
 //    }
 
 
-    @org.junit.Test
+    @Test
     public void testUnsafePut() throws NoSuchFieldException, IllegalAccessException {
         // 通过反射得到theUnsafe对应的Field对象
         Field field = Unsafe.class.getDeclaredField("theUnsafe");
@@ -49,8 +50,18 @@ public void testUnsafePut() throws NoSuchFieldException, IllegalAccessException
 
         System.out.println(user);//打印midified-name,100,101,100.1
     }
-}
 
+    @Test
+    //在指定address写入char。
+    //操作不成功，直接fatal error。
+    public void testPutChar() throws Exception {
+        char s = 'x';
+        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
+        theUnsafe.setAccessible(true);
+        Unsafe unsafe = (Unsafe) theUnsafe.get(null);
+        unsafe.putChar(System.identityHashCode(s), '2');
+    }
+}
 
 class User {
     private String name = "test";

src/java/lang/Integer.java

@@ -611,6 +611,7 @@ private static class IntegerCache {
 
             cache = new Integer[(high - low) + 1];
             int j = low;
+            //将cache[]数组缓存填充满。
             for (int k = 0; k < cache.length; k++)
                 cache[k] = new Integer(j++);
         }

src/sun/misc/Unsafe.java

@@ -204,6 +204,9 @@ public static Unsafe getUnsafe() {
      *
      * @see #putInt(Object, int, int)
      */
+    // o位需要修改的对象
+    // offset为相对对象的偏移地址
+    // x为为该偏移地址上赋的值。
     public native void putObject(Object o, long offset, Object x);
 
     /**