[add] add examples of gc

brianway · brianway · commit 0ebc64aa73a8 · 2017-03-03T16:24:32.000+08:00
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/gc/FinalizeEscapeGC.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/gc/FinalizeEscapeGC.java
@@ -0,0 +1,58 @@
+package com.brianway.learning.java.jvm.gc;
+
+/**
+ * Created by brian on 17/3/3.
+ * 对象可以在被 GC 时自我拯救
+ * 这种自救的机会只有一次,因为一个对象的 finalize() 方法最多只会被系统自动调用一次
+ */
+public class FinalizeEscapeGC {
+    public static FinalizeEscapeGC SAVE_HOOK = null;
+
+    public void isAlive() {
+        System.out.println("yes, i am still alive :)");
+    }
+
+    @Override
+    protected void finalize() throws Throwable {
+        super.finalize();
+        System.out.println("finalize method executed");
+        FinalizeEscapeGC.SAVE_HOOK = this;
+    }
+
+    public static void main(String[] args) throws Throwable {
+        SAVE_HOOK = new FinalizeEscapeGC();
+
+        //对象第一次成功拯救自己
+        SAVE_HOOK = null;
+        System.gc();
+
+        // finalize 优先级低,暂停 0.5 秒以等待
+        Thread.sleep(500);
+
+        if (SAVE_HOOK != null) {
+            SAVE_HOOK.isAlive();
+        } else {
+            System.out.println("no, i ma dead :(");
+        }
+
+        //与上面完全相同,缺失败
+        SAVE_HOOK = null;
+        System.gc();
+
+        // finalize 优先级低,暂停 0.5 秒以等待
+        Thread.sleep(500);
+
+        if (SAVE_HOOK != null) {
+            SAVE_HOOK.isAlive();
+        } else {
+            System.out.println("no, i ma dead :(");
+        }
+
+    }
+}
+
+/*
+finalize method executed
+yes, i am still alive :)
+no, i ma dead :(
+ */
diff --git a/java-jvm/src/main/java/com/brianway/learning/java/jvm/gc/ReferenceCountingGC.java b/java-jvm/src/main/java/com/brianway/learning/java/jvm/gc/ReferenceCountingGC.java
@@ -0,0 +1,31 @@
+package com.brianway.learning.java.jvm.gc;
+
+/**
+ * Created by brian on 17/3/2.
+ * 引用计数算法缺陷
+ */
+public class ReferenceCountingGC {
+    public Object instance = null;
+    private static final int _1MB = 1024 * 1024;
+
+    /**
+     * 占点内存,以便  GC 日志观看
+     */
+    private byte[] bigSize = new byte[2 * _1MB];
+
+    public static void main(String[] args) {
+        testGC();
+    }
+
+    public static void testGC() {
+        ReferenceCountingGC objA = new ReferenceCountingGC();
+        ReferenceCountingGC objB = new ReferenceCountingGC();
+        objA.instance = objB;
+        objB.instance = objA;
+
+        objA = null;
+        objB = null;
+
+        System.gc();
+    }
+}
