org.apache.derby.iapi.services.cache

Class ClassSize

java.lang.Object

org.apache.derby.iapi.services.cache.ClassSize

public class ClassSize
extends java.lang.Object

Field Summary

Fields

Modifier and Type	Field and Description
static int	refSize

Constructor Summary

Constructors

Constructor and Description
ClassSize()

Method Summary

Modifier and Type	Method and Description
static int	estimateAndCatalogBase(java.lang.Class cls) Estimate the static space taken up by a class instance.
static int	estimateArrayOverhead()
static int	estimateBase(java.lang.Class cl) Estimate the static space taken up by the fields of a class.
static int	estimateBaseFromCatalog(java.lang.Class cls) Estimate the static space taken up by a class instance from cataloged coefficients.
static int	estimateBaseFromCoefficients(int[] coeff) Estimate the static space taken up by a class instance given the coefficients returned by getSizeCoefficients.
static int	estimateHashEntrySize() Estimate the size of a Hashtable entry.
static int	estimateMemoryUsage(java.lang.String str) Estimate the size of a string.
static int	getIntSize()
static int	getRefSize() Get the estimate of the size of an object reference.
static int[]	getSizeCoefficients(java.lang.Class cl) The estimate of the size of a class instance depends on whether the JVM uses 32 or 64 bit addresses, that is it depends on the size of an object reference.
static void	setDummyCatalog() do not try to use the catalog.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

refSize

public static final int refSize

Constructor Detail

ClassSize

public ClassSize()

Method Detail

setDummyCatalog

public static void setDummyCatalog()

do not try to use the catalog.

getRefSize

public static int getRefSize()

Get the estimate of the size of an object reference.

Returns:

the estimate in bytes.

getIntSize

public static int getIntSize()

Returns:

the estimate of the size of a primitive int

getSizeCoefficients

public static int[] getSizeCoefficients(java.lang.Class cl)

The estimate of the size of a class instance depends on whether the JVM uses 32 or 64 bit addresses, that is it depends on the size of an object reference. It is a linear function of the size of a reference, e.g. 24 + 5*r where r is the size of a reference (usually 4 or 8 bytes). This method returns the coefficients of the linear function, e.g. {24, 5} in the above example.

Parameters:

cl - A class whose instance size is to be estimated

Returns:

an array of 2 integers. The first integer is the constant part of the function, the second is the reference size coefficient.

estimateBaseFromCoefficients

public static int estimateBaseFromCoefficients(int[] coeff)

Estimate the static space taken up by a class instance given the coefficients returned by getSizeCoefficients.

Parameters:

coeff - the coefficients

Returns:

the size estimate, in bytes

estimateBaseFromCatalog

public static int estimateBaseFromCatalog(java.lang.Class cls)

Estimate the static space taken up by a class instance from cataloged coefficients.

Parameters:

cls - the class

Returns:

the size estimate, in bytes

See Also:

estimateBaseFromCoefficients(int[]), see org.apache.derbyBuild.ClassSizeCrawler

estimateAndCatalogBase

public static int estimateAndCatalogBase(java.lang.Class cls)

Estimate the static space taken up by a class instance. Save the coefficients in a catalog.

Parameters:

cls - the class

Returns:

the size estimate, in bytes

See Also:

estimateBaseFromCoefficients(int[]), see org.apache.derbyBuild.ClassSizeCrawler

estimateBase

public static int estimateBase(java.lang.Class cl)

Estimate the static space taken up by the fields of a class. This includes the space taken up by by references (the pointer) but not by the referenced object. So the estimated size of an array field does not depend on the size of the array. Similarly the size of an object (reference) field does not depend on the object.

Returns:

the size estimate in bytes. Note that this method will throw a SecurityException if the SecurityManager does not let this class execute the method Class.getDeclaredFields(). If this is a concern try to compute the size coefficients at build time. see org.apache.derbyBuild.ClassSizeCrawler

See Also:

estimateBaseFromCatalog(java.lang.Class)

estimateArrayOverhead

public static int estimateArrayOverhead()

Returns:

the estimated overhead of an array. The estimated size of an x[n] array is estimateArrayOverhead() + n*sizeOf(x).

estimateHashEntrySize

public static int estimateHashEntrySize()

Estimate the size of a Hashtable entry. In Java 1.2 we can use Map.entry, but this is not available in earlier versions of Java.

Returns:

the estimate, in bytes

estimateMemoryUsage

public static int estimateMemoryUsage(java.lang.String str)

Estimate the size of a string.

Returns:

the estimated size, in bytes