做一些cache的時候,我們不可能把數據庫的所有的數據都緩存到內存裏面,我們要估計緩存的大小。那麼如何查看java對象所佔的內存大小呢?本文爲大家介紹一下方法,希望可以爲您提供幫助!更多內容請關注應屆畢業生考試網!
(1)做一些cache的時候,我們不可能把數據庫的所有的數據都緩存到內存裏面,我們要估計緩存的大小。
(2)內存泄露的時候,我們可以查看某些對象的大小來定位問題,當然還有其他的更有效的方式,比如使用MAT分析dump文件
(3)根據jvm的堆內存設置,我們可以知道最多可以創建多少個對象。
從jdk5開始,提供了Instrumentation API,它有一個叫做getObjectSize()的方法,但是,這個方法存在兩個問題:
(1)不可以直接使用。必須要實現一個Instrumentation Agent,還得放到jar包裏面。
(2)它只能返回單個對象的大小,不能返回內部包含的.子對象的大小。
關於第一個問題,很好解決,在任何一個類裏面聲明一個"premain"方法,就可以把這個類做成是一個agent:
public class SizeOfAgent {
static Instrumentation inst;
/** initializes agent */
public static void premain(String agentArgs, Instrumentation instP) {
inst = instP;
}
}
jvm在啓動的時候會調用premain()方法,同時會傳遞Instrumentation這個對象實例,要告訴jvm Instrumentation agent所在的類,需要把這個類打到jar包裏面,
然後在這個文件設置一些屬性:
Premain-Class: OfAgent
Boot-Class-Path:
Can-Redefine-Classes: false
java應用在啓動的時候,指定-javaagent參數:
java -javaagent: <Your main class>
拿到Instrumentation這個實例以後,就可以調用sizeOf()方法了:
public class SizeOfAgent {
static Instrumentation inst;
// ...
public static long sizeOf(Object o) {
return bjectSize(o);
}
}
然後可以使用反射來獲取子對象的大小。
完整的代碼如下:
package ;
import rumentation;
import y;
import d;
import fier;
import tityHashMap;
import ;
import k;
public class SizeOfAgent {
static Instrumentation inst;
/** initializes agent */
public static void premain(String agentArgs, Instrumentation instP) {
inst = instP;
}
/**
* Returns object size without member sub-objects.
*
* @param o
* object to get size of
* @return object size
*/
public static long sizeOf(Object o) {
if (inst == null) {
throw new IllegalStateException(
"Can not access instrumentation environment.n"
+ "Please check if jar file containing SizeOfAgent class is n"
+ "specified in the java's "-javaagent" command line argument.");
}
return bjectSize(o);
}
/**
* Calculates full size of object iterating over its hierarchy graph.
*
* @param obj
* object to calculate size of
* @return object size
*/
public static long fullSizeOf(Object obj) {
Map<Object, Object> visited = new IdentityHashMap<Object, Object>();
Stack<Object> stack = new Stack<Object>();
long result = internalSizeOf(obj, stack, visited);
while (!pty()) {
result += internalSizeOf((), stack, visited);
}
r();
return result;
}
private static boolean skipObject(Object obj, Map<Object, Object> visited) {
if (obj instanceof String) {
// skip interned string
if (obj == ((String) obj)rn()) {
return true;
}
}
return (obj == null) // skip visited object
|| ainsKey(obj);
}
private static long internalSizeOf(Object obj, Stack<Object> stack,
Map<Object, Object> visited) {
if (skipObject(obj, visited)) {
return 0;
}
(obj, null);
long result = 0;
// get size of object + primitive variables + member pointers
result += Of(obj);
// process all array elements
Class clazz = lass();
if (ray()) {
if (ame()th() != 2) {// skip primitive type array
int length = ength(obj);
for (int i = 0; i < length; i++) {
((obj, i));
}
}
return result;
}
// process all fields of the object
while (clazz != null) {
Field[] fields = eclaredFields();
for (int i = 0; i < th; i++) {
if (!atic(fields[i]odifiers())) {
if (fields[i]ype()imitive()) {
continue; // skip primitive fields
} else {
fields[i]ccessible(true);
try {
// objects to be estimated are put to stack
Object objectToAdd = fields[i](obj);
if (objectToAdd != null) {
(objectToAdd);
}
} catch (IllegalAccessException ex) {
assert false;
}
}
}
}
clazz = uperclass();
}
return result;
}
}
然後我們可以做一個測試:
public class Test {
static class Person{
private int id;
private String name;
private String address;
public Person(int id, String name, String address) {
= id;
= name;
ess = address;
}
}
public static void main(String[] args) throws Exception {
Person p = new Person(12, "xujsh","bj");
long size = SizeOf(p);
tln(size);
}
}
切換到命令行:
D:workspaceobjsizesrc>java -version
java version "1.6.0_22"
Java(TM) SE Runtime Environment (build 1.6.0_22-b04)
Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing)
D:workspaceobjsizesrc>javac com/bj58/test/*
D:workspaceobjsizesrc>jar -cvfm com/bj58/test/*
標明清單(manifest)
增加:com/bj58/test/s(讀入= 3119) (寫出= 1698)(壓縮了 45%)
增加:com/bj58/test/(讀入= 3147) (寫出= 1204)(壓縮了 61%)
增加:com/bj58/test/Test$s(讀入= 442) (寫出= 305)(壓縮了 30%)
增加:com/bj58/test/s(讀入= 692) (寫出= 441)(壓縮了 36%)
增加:com/bj58/test/(讀入= 509) (寫出= 290)(壓縮了 43%)
D:workspaceobjsizesrc>java -javaagent:
24
:
Manifest-Version: 1.0
Main-Class:
Premain-Class: OfAgent
Boot-Class-Path:
Can-Redefine-Classes: false
【注意】文件的格式要求比較嚴格,每一行要滿足:key:空格value回車
如何在web應用程序裏面使用呢?
以我的tomcat爲例,
(1)把上傳tomcat的lib目錄下面
(2)修改:
添加一行:
JAVA_OPTS="$JAVA_OPTS -javaagent:$CATALINA_HOME/lib/" //這一行是新添加的
if [ -z "$LOGGING_MANAGER" ]; then
JAVA_OPTS="$JAVA_OPTS ger=sLoaderLogManager"
else
JAVA_OPTS="$JAVA_OPTS $LOGGING_MANAGER"
fi
(3)在應用裏面添加一個controler:
@Path(value = "/api/size")
@GET
public ActionResult size() {
Map<Long, List<Long>> map = otindexBaidu();
long size = SizeOf(map);
return new ApiActionResult("size:"+size);
}