jvm基礎知識1
堆放實例對象，棧放實例對象的引用，方法區存儲創建類的信息
上面堆和垃圾回收的關系，垃圾回收回收的是堆內存的數據，s0和s1區域，
例如現在我們要清除s0中的堆對象，將s0中正在運行的對象從s0區域移動到s1區域中，然後將s0中要被清除的堆對象清除，現在對象都存儲在s1中，s0中為空，即下來要清除s1中的對象，將不被清除的對象從s1中移動到s0中，將s1中的對象清除，依次類推
接下來學習下虛擬機的參數：
1、第一點給堆設置參數
-Xms5m -Xmx20m
初始化堆大小5m，最大設置為20M,-XX:+PrintCommandLineFlags將配置的參數從控制臺打印輸出
配置參數：
我們來看下面代碼的運行結果：
package com.bjsxt.base001; public class Test01 { public static void main(String[] args) { //-Xms5m -Xmx20m -XX:+PrintGCDetails -XX:+UseSerialGC -XX:+PrintCommandLineFlags //查看GC信息 System.out.println("max memory:" + Runtime.getRuntime().maxMemory()); System.out.println("free memory:" + Runtime.getRuntime().freeMemory()); System.out.println("total memory:" + Runtime.getRuntime().totalMemory()); byte[] b1 = new byte[1*1024*1024]; System.out.println("分配了1M"); System.out.println("max memory:" + Runtime.getRuntime().maxMemory()); System.out.println("free memory:" + Runtime.getRuntime().freeMemory()); System.out.println("total memory:" + Runtime.getRuntime().totalMemory()); byte[] b2 = new byte[4*1024*1024]; System.out.println("分配了4M"); System.out.println("max memory:" + Runtime.getRuntime().maxMemory()); System.out.println("free memory:" + Runtime.getRuntime().freeMemory()); System.out.println("total memory:" + Runtime.getRuntime().totalMemory()); } }
-XX:InitialHeapSize=5242880 -XX:MaxHeapSize=20971520 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseSerialGC
max memory:20316160
free memory:4418752
total memory:5111808
[GC[DefNew: 676K->127K(1536K), 0.0014042 secs] 676K->481K(4992K), 0.0014266 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
分配了1M
max memory:20316160
free memory:3541280
total memory:5111808
[GC[DefNew: 1180K->0K(1536K), 0.0010578 secs][Tenured: 1505K->1505K(3456K), 0.0022471 secs] 1533K->1505K(4992K), [Perm : 2511K->2511K(21248K)], 0.0033312 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
分配了4M
max memory:20316160
free memory:3625944
total memory:9441280
Heap
def new generation total 1664K, used 108K [0x00000000f9a00000, 0x00000000f9bc0000, 0x00000000fa0a0000)
eden space 1536K, 7% used [0x00000000f9a00000, 0x00000000f9a1b100, 0x00000000f9b80000)
from space 128K, 0% used [0x00000000f9b80000, 0x00000000f9b80000, 0x00000000f9ba0000)
to space 128K, 0% used [0x00000000f9ba0000, 0x00000000f9ba0000, 0x00000000f9bc0000)
tenured generation total 7556K, used 5601K [0x00000000fa0a0000, 0x00000000fa801000, 0x00000000fae00000)
the space 7556K, 74% used [0x00000000fa0a0000, 0x00000000fa618670, 0x00000000fa618800, 0x00000000fa801000)
compacting perm gen total 21248K, used 2520K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
the space 21248K, 11% used [0x00000000fae00000, 0x00000000fb076370, 0x00000000fb076400, 0x00000000fc2c0000)
No shared spaces configured.
[GC[DefNew: 1180K->0K(1536K), 0.0010578 secs][Tenured: 1505K->1505K(3456K), 0.0022471 secs] 1533K->1505K(4992K), [Perm : 2511K->2511K(21248K)], 0.0033312 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
表示進行了垃圾回收，DefNew表示回收了新生代，Tenured表示回收了老年代，Perm 表示回收了永久區
新生代會頻繁被GC不穩定，新生代包含eden空間 s1 和s0空間
我們來看下面的列子：
package com.bjsxt.base001; public class Test02 { public static void main(String[] args) { //第一次配置 //-Xms20m -Xmx20m -Xmn1m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC //第二次配置 //-Xms20m -Xmx20m -Xmn7m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC //第三次配置 //-XX:NewRatio=老年代/新生代 //-Xms20m -Xmx20m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC byte[] b = null; //連續向系統申請10MB空間 for(int i = 0 ; i <10; i ++){ b = new byte[1*1024*1024]; } } }
程序運行的效果是：
[GC[DefNew: 512K->255K(768K), 0.0013618 secs] 512K->439K(20224K), 0.0013894 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 768K, used 428K [0x00000000f9a00000, 0x00000000f9b00000, 0x00000000f9b00000)
eden space 512K, 33% used [0x00000000f9a00000, 0x00000000f9a2b300, 0x00000000f9a80000)
from space 256K, 99% used [0x00000000f9ac0000, 0x00000000f9affff8, 0x00000000f9b00000)
to space 256K, 0% used [0x00000000f9a80000, 0x00000000f9a80000, 0x00000000f9ac0000)
tenured generation total 19456K, used 10424K [0x00000000f9b00000, 0x00000000fae00000, 0x00000000fae00000)
the space 19456K, 53% used [0x00000000f9b00000, 0x00000000fa52e040, 0x00000000fa52e200, 0x00000000fae00000)
compacting perm gen total 21248K, used 2517K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
the space 21248K, 11% used [0x00000000fae00000, 0x00000000fb075440, 0x00000000fb075600, 0x00000000fc2c0000)
No shared spaces configured.
eden space 512K，from space 256K是 比例是2，SurvivorRatio=2 就是這個含義設置的意思 xmn表示新生代的值設置成1M，from區域就是s0區域，to區域就是s1區域，二者的大小是一樣的
eden space 512K, 33% used [0x00000000f9a00000, 0x00000000f9a2b300, 0x00000000f9a80000)
from space 256K, 99% used [0x00000000f9ac0000, 0x00000000f9affff8, 0x00000000f9b00000)
to space 256K, 0% used [0x00000000f9a80000, 0x00000000f9a80000, 0x00000000f9ac0000)
上面三者之和就是1M
真正在應用的場景中，已經盡量減少GC，將初始化的內存大小和最大內存大小配置成一樣，這樣可以減少GC
package com.bjsxt.base001; import java.util.Vector; public class Test03 { public static void main(String[] args) { //-Xms1m -Xmx1m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d:/Test03.dump //堆內存溢出 Vector v = new Vector(); for(int i=0; i < 5; i ++){ v.add(new Byte[1*1024*1024]); } } }
上面這個代碼我申請5M的內存，但是在設置的時候卻是
/-Xms1m -Xmx1m 設置成1M，就會出現堆內存異常，使用-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath就會將信息導出來查看
package com.bjsxt.base001; public class Test04 { //-Xss1m //-Xss5m //棧調用深度 private static int count; public static void recursion(){ count++; recursion(); } public static void main(String[] args){ try { recursion(); } catch (Throwable t) { System.out.println("調用最大深入：" + count); t.printStackTrace(); } } }
運行結果：
調用最大深入：12560
java.lang.StackOverflowError
jvm基礎知識1