Update Java1.md

Java1.md

@@ -48,4 +48,11 @@
 ### JVM
 Eden so s1 8:1:1
 由如下参数控制提升阈值 -XX:+MaxTenuringThreshold=15
-
+mark-and-sweep algorithm.  
+   The algorithm traverses all object references, starting with the GC roots, and marks every object found as alive.  
+   All of the heap memory that is not occupied by marked objects is reclaimed. It is simply marked as free, essentially swept free of unused objects.
+可以作为 GC Roots 的对象
+1. 当前正在执行的方法里的局部变量和输入参数  
+2. 活动线程(Active threads)  
+3. 所有类的静态字段(static field) 
+4. JNI 引用