Java 基本数据类型 sizeof 功能

Java基本数据类型 int     32bit short   16bit long    64bit byte    8bit

char    16bit float   32bit double  64bit

boolean 1bit,This data type represents one bit of information, but its "size" isn't something that's precisely defined.(ref

Java基本数据类型大小

	private static void calSize() {
		System.out.println("Integer: " + Integer.SIZE/8);			// 4
		System.out.println("Short: " + Short.SIZE/8);				// 2	
		System.out.println("Long: " + Long.SIZE/8);					// 8
		System.out.println("Byte: " + Byte.SIZE/8);					// 1
		System.out.println("Character: " + Character.SIZE/8);		// 2
		System.out.println("Float: " + Float.SIZE/8);				// 4
		System.out.println("Double: " + Double.SIZE/8);				// 8
//		System.out.println("Boolean: " + Boolean);
	}

Java中模拟c中对sizeof的实现

思路:利用java中GC内存回收前后的heap size差别,得出每个object的大小

这是一个程序,java中没有现成的sizeof的实现,原因主要是java中的基本数据类型的大小都是固定的,所以看上去没有必要用sizeof这个关键字。实现的想法是这样的:java.lang.Runtime类中有一些简单的能涉及到内存管理的函数:Every Java application has a single instance of class Runtime that allows the application to interface with the environment in which the application is running. The current runtime can be obtained from the getRuntime method. longfreeMemory()Returns the amount of free memory in the Java Virtual Machine. voidgc()Runs the garbage collector.static RuntimegetRuntime()Returns the runtime object associated with the current Java application. longmaxMemory()Returns the maximum amount of memory that the Java virtual machine will attempt to use. voidrunFinalization()Runs the finalization methods of any objects pending finalization.使用这些简单的内存访问,可以得到内存的一些情况,我们通过建立一个大的某个类的数组,来查看内存用了多少,进而可以求得类的大小。源码: private static void calSize2() { runGC(); long heap1 = 0; final int count = 100000; Object[] objs = new Object[count]; for(int i=-1; i<count; i++) { Object obj = null; obj = new Object(); // 8 // obj = new Integer( i ); // 16 // obj = new Short( (short)i ); // 16 // obj = new Long( i ); // 16 // obj = new Byte( (byte)0 ); // 16 // obj = new Character( (char)i ); // 16 // obj = new Float( i ); // 16 // obj = new Double( i ); // 16 // obj = new Boolean( true ); // 16 // obj = new String(); // 40 if(i<0){ obj = null; runGC(); heap1 = usedMemory(); // before memory size } else { objs[i] = obj; } } runGC(); long heap2 = usedMemory(); // after memory size final int size = (int)Math.round( (heap2 - heap1)/(double)count ); System.out.println("heap1 = " + heap1 + "; heap2 = " + heap2); System.out.println("heap2-heap1 = " + (heap2 - heap1) + "; " + objs[0].getClass().getSimpleName() + " size = " + size); for(int i=0; i<count; i++) { objs[i] = null; } objs = null; runGC(); } private static void runGC() { for(int i=0; i<4; i++) { long usedMem1 = usedMemory(); long usedMem2 = Long.MAX_VALUE; for(int j=0; (usedMem1<usedMem2) && (j<500); j++) { rTime.runFinalization(); rTime.gc(); Thread.yield(); usedMem2 = usedMem1; usedMem1 = usedMemory(); } } } private static long usedMemory() { return rTime.totalMemory() - rTime.freeMemory(); }注意:Object[] objects = new Object[count];只是分配了数组空间,没有分配对象的空间。数组中只有引用而已。 结论:下代码测试基本对象时,得出的结果象下面:   Object obj = null;obj = new Object(); // 8obj = new Integer( i ); // 16obj = new Short( (short)i ); // 16obj = new Long( i ); // 16obj = new Byte( (byte)0 ); // 16obj = new Character( (char)i ); // 16obj = new Float( i ); // 16obj = new Double( i ); // 16obj = new Boolean( true ); // 16obj = new String(); // 40怎么会这样呢???解释如下: 这个例子写的很好,正好说明了java中基本类型封装对象所占内存的大小.   1.简单的Object对象要占用8个字节的内存空间,因为每个实例都至少必须包含一些最基本操作,比如:wait()/notify(),equals(),   hashCode()等   2.使用Integer对象占用了16个字节,而int占用4个字节,说了封装了之后内存消耗大了4倍   3.那么Long看起来比Integer对象应该使用更多空间,结果Long所占的空间也是16个字节.   那么就正好说明了JVM的对于基本类型封装对象的内存分配的规则是如下:   Object所占内存(8个字节)+最大基本类型(long)所占内存(8个字节)   =   16字节.   JVM强制使用8个字节作为边界.   所以所有基本类型封装对象所占内存的大小都是16字节.但是还是有区别,比如:Integer对象虽然占用了16个字节的内存,但是只是利用了 Object所占内存(8个字节)+int所占内存(4个字节)   =   12字节.还有4个字节根本没有被使用.呵呵,仔细分析了一晚,还是有很多收获的测试源码下载参考推荐:关于java中boolean占用字节的问题java中的 boolean 在内存中占多少字节?Primitive Data Types (SUN 官方文档)

long

freeMemory()Returns the amount of free memory in the Java Virtual Machine.

void

gc()Runs the garbage collector.

static Runtime

getRuntime()Returns the runtime object associated with the current Java application.

long

maxMemory()Returns the maximum amount of memory that the Java virtual machine will attempt to use.

void

runFinalization()Runs the finalization methods of any objects pending finalization.

long

freeMemory()Returns the amount of free memory in the Java Virtual Machine.

void

gc()Runs the garbage collector.

static Runtime

getRuntime()Returns the runtime object associated with the current Java application.

long

maxMemory()Returns the maximum amount of memory that the Java virtual machine will attempt to use.

void

runFinalization()Runs the finalization methods of any objects pending finalization.

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