public abstract class XMLSignatureFactory extends Object
XMLSignature objects from scratch or
for unmarshalling an XMLSignature object from a corresponding
XML representation.
XMLSignatureFactory Type
Each instance of XMLSignatureFactory supports a specific
XML mechanism type. To create an XMLSignatureFactory, call one
of the static getInstance methods, passing in the XML
mechanism type desired, for example:
XMLSignatureFactory factory = XMLSignatureFactory.getInstance("DOM");
The objects that this factory produces will be based
on DOM and abide by the DOM interoperability requirements as defined in the
DOM Mechanism Requirements.
See the XMLSignatureFactory section in the
Java Security Standard Algorithm Names Specification for a list of
standard mechanism types.
XMLSignatureFactory implementations are registered and loaded
using the Provider mechanism.
For example, a service provider that supports the
DOM mechanism would be specified in the Provider subclass as:
put("XMLSignatureFactory.DOM", "org.example.DOMXMLSignatureFactory");
An implementation MUST minimally support the default mechanism type: DOM.
Note that a caller must use the same XMLSignatureFactory
instance to create the XMLStructures of a particular
XMLSignature that is to be generated. The behavior is
undefined if XMLStructures from different providers or
different mechanism types are used together.
Also, the XMLStructures that are created by this factory
may contain state specific to the XMLSignature and are not
intended to be reusable.
Creating XMLSignatures from scratch
Once the XMLSignatureFactory has been created, objects
can be instantiated by calling the appropriate method. For example, a
Reference instance may be created by invoking one of the
newReference methods.
Unmarshalling XMLSignatures from XML
Alternatively, an XMLSignature may be created from an
existing XML representation by invoking the unmarshalXMLSignature method and passing it a mechanism-specific
XMLValidateContext instance containing the XML content:
DOMValidateContext context = new DOMValidateContext(key, signatureElement); XMLSignature signature = factory.unmarshalXMLSignature(context);Each
XMLSignatureFactory must support the required
XMLValidateContext types for that factory type, but may support
others. A DOM XMLSignatureFactory must support DOMValidateContext objects.
Signing and marshalling XMLSignatures to XML
EachXMLSignature created by the factory can also be
marshalled to an XML representation and signed, by invoking the
sign method of the
XMLSignature object and passing it a mechanism-specific
XMLSignContext object containing the signing key and
marshalling parameters (see DOMSignContext).
For example:
DOMSignContext context = new DOMSignContext(privateKey, document);
signature.sign(context);
Concurrent Access
The static methods of this class are guaranteed to be thread-safe. Multiple threads may concurrently invoke the static methods defined in this class with no ill effects.
However, this is not true for the non-static methods defined by this
class. Unless otherwise documented by a specific provider, threads that
need to access a single XMLSignatureFactory instance
concurrently should synchronize amongst themselves and provide the
necessary locking. Multiple threads each manipulating a different
XMLSignatureFactory instance need not synchronize.
- Since:
- 1.6
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Constructor Summary
Constructors Modifier Constructor Description protectedXMLSignatureFactory()Default constructor, for invocation by subclasses. -
Method Summary
Modifier and Type Method Description static XMLSignatureFactorygetInstance()Returns anXMLSignatureFactorythat supports the default XML processing mechanism and representation type ("DOM").static XMLSignatureFactorygetInstance(String mechanismType)Returns anXMLSignatureFactorythat supports the specified XML processing mechanism and representation type (ex: "DOM").static XMLSignatureFactorygetInstance(String mechanismType, String provider)Returns anXMLSignatureFactorythat supports the requested XML processing mechanism and representation type (ex: "DOM"), as supplied by the specified provider.static XMLSignatureFactorygetInstance(String mechanismType, Provider provider)Returns anXMLSignatureFactorythat supports the requested XML processing mechanism and representation type (ex: "DOM"), as supplied by the specified provider.KeyInfoFactorygetKeyInfoFactory()Returns aKeyInfoFactorythat createsKeyInfoobjects.StringgetMechanismType()Returns the type of the XML processing mechanism and representation supported by thisXMLSignatureFactory(ex: "DOM").ProvidergetProvider()Returns the provider of thisXMLSignatureFactory.abstract URIDereferencergetURIDereferencer()Returns a reference to theURIDereferencerthat is used by default to dereference URIs inReferenceobjects.abstract booleanisFeatureSupported(String feature)Indicates whether a specified feature is supported.abstract CanonicalizationMethodnewCanonicalizationMethod(String algorithm, C14NMethodParameterSpec params)Creates aCanonicalizationMethodfor the specified algorithm URI and parameters.abstract CanonicalizationMethodnewCanonicalizationMethod(String algorithm, XMLStructure params)Creates aCanonicalizationMethodfor the specified algorithm URI and parameters.abstract DigestMethodnewDigestMethod(String algorithm, DigestMethodParameterSpec params)Creates aDigestMethodfor the specified algorithm URI and parameters.abstract ManifestnewManifest(List<? extends Reference> references)Creates aManifestcontaining the specified list ofReferences.abstract ManifestnewManifest(List<? extends Reference> references, String id)Creates aManifestcontaining the specified list ofReferences and optional id.abstract ReferencenewReference(String uri, DigestMethod dm)Creates aReferencewith the specified URI and digest method.abstract ReferencenewReference(String uri, DigestMethod dm, List<? extends Transform> transforms, String type, String id)Creates aReferencewith the specified parameters.abstract ReferencenewReference(String uri, DigestMethod dm, List<? extends Transform> transforms, String type, String id, byte[] digestValue)Creates aReferencewith the specified parameters and pre-calculated digest value.abstract ReferencenewReference(String uri, DigestMethod dm, List<? extends Transform> appliedTransforms, Data result, List<? extends Transform> transforms, String type, String id)Creates aReferencewith the specified parameters.abstract SignatureMethodnewSignatureMethod(String algorithm, SignatureMethodParameterSpec params)Creates aSignatureMethodfor the specified algorithm URI and parameters.abstract SignaturePropertiesnewSignatureProperties(List<? extends SignatureProperty> properties, String id)Creates aSignaturePropertiescontaining the specified list ofSignaturePropertys and optional id.abstract SignaturePropertynewSignatureProperty(List<? extends XMLStructure> content, String target, String id)Creates aSignaturePropertycontaining the specified list ofXMLStructures, target URI and optional id.abstract SignedInfonewSignedInfo(CanonicalizationMethod cm, SignatureMethod sm, List<? extends Reference> references)Creates aSignedInfowith the specified canonicalization and signature methods, and list of one or more references.abstract SignedInfonewSignedInfo(CanonicalizationMethod cm, SignatureMethod sm, List<? extends Reference> references, String id)Creates aSignedInfowith the specified parameters.abstract TransformnewTransform(String algorithm, TransformParameterSpec params)Creates aTransformfor the specified algorithm URI and parameters.abstract TransformnewTransform(String algorithm, XMLStructure params)Creates aTransformfor the specified algorithm URI and parameters.abstract XMLObjectnewXMLObject(List<? extends XMLStructure> content, String id, String mimeType, String encoding)Creates anXMLObjectfrom the specified parameters.abstract XMLSignaturenewXMLSignature(SignedInfo si, KeyInfo ki)Creates anXMLSignatureand initializes it with the contents of the specifiedSignedInfoandKeyInfoobjects.abstract XMLSignaturenewXMLSignature(SignedInfo si, KeyInfo ki, List<? extends XMLObject> objects, String id, String signatureValueId)Creates anXMLSignatureand initializes it with the specified parameters.abstract XMLSignatureunmarshalXMLSignature(XMLValidateContext context)Unmarshals a newXMLSignatureinstance from a mechanism-specificXMLValidateContextinstance.abstract XMLSignatureunmarshalXMLSignature(XMLStructure xmlStructure)Unmarshals a newXMLSignatureinstance from a mechanism-specificXMLStructureinstance.
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Constructor Details
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XMLSignatureFactory
protected XMLSignatureFactory()Default constructor, for invocation by subclasses.
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Method Details
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getInstance
Returns anXMLSignatureFactorythat supports the specified XML processing mechanism and representation type (ex: "DOM").This method uses the standard JCA provider lookup mechanism to locate and instantiate an
XMLSignatureFactoryimplementation of the desired mechanism type. It traverses the list of registered securityProviders, starting with the most preferredProvider. A newXMLSignatureFactoryobject from the firstProviderthat supports the specified mechanism is returned.Note that the list of registered providers may be retrieved via the
Security.getProviders()method.- Implementation Note:
- The JDK Reference Implementation additionally uses the
jdk.security.provider.preferredSecurityproperty to determine the preferred provider order for the specified algorithm. This may be different than the order of providers returned bySecurity.getProviders(). - Parameters:
mechanismType- the type of the XML processing mechanism and representation. See theXMLSignatureFactorysection in the Java Security Standard Algorithm Names Specification for a list of standard mechanism types.- Returns:
- a new
XMLSignatureFactory - Throws:
NullPointerException- ifmechanismTypeisnullNoSuchMechanismException- if noProvidersupports anXMLSignatureFactoryimplementation for the specified mechanism- See Also:
Provider
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getInstance
Returns anXMLSignatureFactorythat supports the requested XML processing mechanism and representation type (ex: "DOM"), as supplied by the specified provider. Note that the specifiedProviderobject does not have to be registered in the provider list.- Parameters:
mechanismType- the type of the XML processing mechanism and representation. See theXMLSignatureFactorysection in the Java Security Standard Algorithm Names Specification for a list of standard mechanism types.provider- theProviderobject- Returns:
- a new
XMLSignatureFactory - Throws:
NullPointerException- ifproviderormechanismTypeisnullNoSuchMechanismException- if anXMLSignatureFactoryimplementation for the specified mechanism is not available from the specifiedProviderobject- See Also:
Provider
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getInstance
public static XMLSignatureFactory getInstance(String mechanismType, String provider) throws NoSuchProviderExceptionReturns anXMLSignatureFactorythat supports the requested XML processing mechanism and representation type (ex: "DOM"), as supplied by the specified provider. The specified provider must be registered in the security provider list.Note that the list of registered providers may be retrieved via the
Security.getProviders()method.- Parameters:
mechanismType- the type of the XML processing mechanism and representation. See theXMLSignatureFactorysection in the Java Security Standard Algorithm Names Specification for a list of standard mechanism types.provider- the string name of the provider- Returns:
- a new
XMLSignatureFactory - Throws:
NoSuchProviderException- if the specified provider is not registered in the security provider listNullPointerException- ifproviderormechanismTypeisnullNoSuchMechanismException- if anXMLSignatureFactoryimplementation for the specified mechanism is not available from the specified provider- See Also:
Provider
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getInstance
Returns anXMLSignatureFactorythat supports the default XML processing mechanism and representation type ("DOM").This method uses the standard JCA provider lookup mechanism to locate and instantiate an
XMLSignatureFactoryimplementation of the default mechanism type. It traverses the list of registered securityProviders, starting with the most preferredProvider. A newXMLSignatureFactoryobject from the firstProviderthat supports the DOM mechanism is returned.Note that the list of registered providers may be retrieved via the
Security.getProviders()method.- Returns:
- a new
XMLSignatureFactory - Throws:
NoSuchMechanismException- if noProvidersupports anXMLSignatureFactoryimplementation for the DOM mechanism- See Also:
Provider
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getMechanismType
Returns the type of the XML processing mechanism and representation supported by thisXMLSignatureFactory(ex: "DOM").- Returns:
- the XML processing mechanism type supported by this
XMLSignatureFactory
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getProvider
Returns the provider of thisXMLSignatureFactory.- Returns:
- the provider of this
XMLSignatureFactory
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newXMLSignature
Creates anXMLSignatureand initializes it with the contents of the specifiedSignedInfoandKeyInfoobjects.- Parameters:
si- the signed infoki- the key info (may benull)- Returns:
- an
XMLSignature - Throws:
NullPointerException- ifsiisnull
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newXMLSignature
public abstract XMLSignature newXMLSignature(SignedInfo si, KeyInfo ki, List<? extends XMLObject> objects, String id, String signatureValueId)Creates anXMLSignatureand initializes it with the specified parameters.- Parameters:
si- the signed infoki- the key info (may benull)objects- a list ofXMLObjects (may be empty ornull)id- the Id (may benull)signatureValueId- the SignatureValue Id (may benull)- Returns:
- an
XMLSignature - Throws:
NullPointerException- ifsiisnullClassCastException- if any of theobjectsare not of typeXMLObject
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newReference
Creates aReferencewith the specified URI and digest method.- Parameters:
uri- the reference URI (may benull)dm- the digest method- Returns:
- a
Reference - Throws:
IllegalArgumentException- ifuriis not RFC 2396 compliantNullPointerException- ifdmisnull
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newReference
public abstract Reference newReference(String uri, DigestMethod dm, List<? extends Transform> transforms, String type, String id)Creates aReferencewith the specified parameters.- Parameters:
uri- the reference URI (may benull)dm- the digest methodtransforms- a list ofTransforms. The list is defensively copied to protect against subsequent modification. May benullor empty.type- the reference type, as a URI (may benull)id- the reference ID (may benull)- Returns:
- a
Reference - Throws:
ClassCastException- if any of thetransformsare not of typeTransformIllegalArgumentException- ifuriis not RFC 2396 compliantNullPointerException- ifdmisnull
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newReference
public abstract Reference newReference(String uri, DigestMethod dm, List<? extends Transform> transforms, String type, String id, byte[] digestValue)Creates aReferencewith the specified parameters and pre-calculated digest value.This method is useful when the digest value of a
Referencehas been previously computed. See for example, the OASIS-DSS (Digital Signature Services) specification.- Parameters:
uri- the reference URI (may benull)dm- the digest methodtransforms- a list ofTransforms. The list is defensively copied to protect against subsequent modification. May benullor empty.type- the reference type, as a URI (may benull)id- the reference ID (may benull)digestValue- the digest value. The array is cloned to protect against subsequent modification.- Returns:
- a
Reference - Throws:
ClassCastException- if any of thetransformsare not of typeTransformIllegalArgumentException- ifuriis not RFC 2396 compliantNullPointerException- ifdmordigestValueisnull
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newReference
public abstract Reference newReference(String uri, DigestMethod dm, List<? extends Transform> appliedTransforms, Data result, List<? extends Transform> transforms, String type, String id)Creates aReferencewith the specified parameters.This method is useful when a list of transforms have already been applied to the
Reference. See for example, the OASIS-DSS (Digital Signature Services) specification.When an
XMLSignaturecontaining this reference is generated, the specifiedtransforms(if non-null) are applied to the specifiedresult. TheTransformselement of the resultingReferenceelement is set to the concatenation of theappliedTransformsandtransforms.- Parameters:
uri- the reference URI (may benull)dm- the digest methodappliedTransforms- a list ofTransforms that have already been applied. The list is defensively copied to protect against subsequent modification. The list must contain at least one entry.result- the result of processing the sequence ofappliedTransformstransforms- a list ofTransforms that are to be applied when generating the signature. The list is defensively copied to protect against subsequent modification. May benullor empty.type- the reference type, as a URI (may benull)id- the reference ID (may benull)- Returns:
- a
Reference - Throws:
ClassCastException- if any of the transforms (in either list) are not of typeTransformIllegalArgumentException- ifuriis not RFC 2396 compliant orappliedTransformsis emptyNullPointerException- ifdm,appliedTransformsorresultisnull
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newSignedInfo
public abstract SignedInfo newSignedInfo(CanonicalizationMethod cm, SignatureMethod sm, List<? extends Reference> references)Creates aSignedInfowith the specified canonicalization and signature methods, and list of one or more references.- Parameters:
cm- the canonicalization methodsm- the signature methodreferences- a list of one or moreReferences. The list is defensively copied to protect against subsequent modification.- Returns:
- a
SignedInfo - Throws:
ClassCastException- if any of the references are not of typeReferenceIllegalArgumentException- ifreferencesis emptyNullPointerException- if any of the parameters arenull
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newSignedInfo
public abstract SignedInfo newSignedInfo(CanonicalizationMethod cm, SignatureMethod sm, List<? extends Reference> references, String id)Creates aSignedInfowith the specified parameters.- Parameters:
cm- the canonicalization methodsm- the signature methodreferences- a list of one or moreReferences. The list is defensively copied to protect against subsequent modification.id- the id (may benull)- Returns:
- a
SignedInfo - Throws:
ClassCastException- if any of the references are not of typeReferenceIllegalArgumentException- ifreferencesis emptyNullPointerException- ifcm,sm, orreferencesarenull
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newXMLObject
public abstract XMLObject newXMLObject(List<? extends XMLStructure> content, String id, String mimeType, String encoding)Creates anXMLObjectfrom the specified parameters.- Parameters:
content- a list ofXMLStructures. The list is defensively copied to protect against subsequent modification. May benullor empty.id- the Id (may benull)mimeType- the mime type (may benull)encoding- the encoding (may benull)- Returns:
- an
XMLObject - Throws:
ClassCastException- ifcontentcontains any entries that are not of typeXMLStructure
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newManifest
Creates aManifestcontaining the specified list ofReferences.- Parameters:
references- a list of one or moreReferences. The list is defensively copied to protect against subsequent modification.- Returns:
- a
Manifest - Throws:
NullPointerException- ifreferencesisnullIllegalArgumentException- ifreferencesis emptyClassCastException- ifreferencescontains any entries that are not of typeReference
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newManifest
Creates aManifestcontaining the specified list ofReferences and optional id.- Parameters:
references- a list of one or moreReferences. The list is defensively copied to protect against subsequent modification.id- the id (may benull)- Returns:
- a
Manifest - Throws:
NullPointerException- ifreferencesisnullIllegalArgumentException- ifreferencesis emptyClassCastException- ifreferencescontains any entries that are not of typeReference
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newSignatureProperty
public abstract SignatureProperty newSignatureProperty(List<? extends XMLStructure> content, String target, String id)Creates aSignaturePropertycontaining the specified list ofXMLStructures, target URI and optional id.- Parameters:
content- a list of one or moreXMLStructures. The list is defensively copied to protect against subsequent modification.target- the target URI of the Signature that this property applies toid- the id (may benull)- Returns:
- a
SignatureProperty - Throws:
NullPointerException- ifcontentortargetisnullIllegalArgumentException- ifcontentis emptyClassCastException- ifcontentcontains any entries that are not of typeXMLStructure
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newSignatureProperties
public abstract SignatureProperties newSignatureProperties(List<? extends SignatureProperty> properties, String id)Creates aSignaturePropertiescontaining the specified list ofSignaturePropertys and optional id.- Parameters:
properties- a list of one or moreSignaturePropertys. The list is defensively copied to protect against subsequent modification.id- the id (may benull)- Returns:
- a
SignatureProperties - Throws:
NullPointerException- ifpropertiesisnullIllegalArgumentException- ifpropertiesis emptyClassCastException- ifpropertiescontains any entries that are not of typeSignatureProperty
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newDigestMethod
public abstract DigestMethod newDigestMethod(String algorithm, DigestMethodParameterSpec params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aDigestMethodfor the specified algorithm URI and parameters.- Parameters:
algorithm- the URI identifying the digest algorithmparams- algorithm-specific digest parameters (may benull)- Returns:
- the
DigestMethod - Throws:
InvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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newSignatureMethod
public abstract SignatureMethod newSignatureMethod(String algorithm, SignatureMethodParameterSpec params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aSignatureMethodfor the specified algorithm URI and parameters.- Parameters:
algorithm- the URI identifying the signature algorithmparams- algorithm-specific signature parameters (may benull)- Returns:
- the
SignatureMethod - Throws:
InvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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newTransform
public abstract Transform newTransform(String algorithm, TransformParameterSpec params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aTransformfor the specified algorithm URI and parameters.- Parameters:
algorithm- the URI identifying the transform algorithmparams- algorithm-specific transform parameters (may benull)- Returns:
- the
Transform - Throws:
InvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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newTransform
public abstract Transform newTransform(String algorithm, XMLStructure params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aTransformfor the specified algorithm URI and parameters. The parameters are specified as a mechanism-specificXMLStructure(ex:DOMStructure). This method is useful when the parameters are in XML form or there is no standard class for specifying the parameters.- Parameters:
algorithm- the URI identifying the transform algorithmparams- a mechanism-specific XML structure from which to unmarshal the parameters from (may benullif not required or optional)- Returns:
- the
Transform - Throws:
ClassCastException- if the type ofparamsis inappropriate for thisXMLSignatureFactoryInvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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newCanonicalizationMethod
public abstract CanonicalizationMethod newCanonicalizationMethod(String algorithm, C14NMethodParameterSpec params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aCanonicalizationMethodfor the specified algorithm URI and parameters.- Parameters:
algorithm- the URI identifying the canonicalization algorithmparams- algorithm-specific canonicalization parameters (may benull)- Returns:
- the
CanonicalizationMethod - Throws:
InvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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newCanonicalizationMethod
public abstract CanonicalizationMethod newCanonicalizationMethod(String algorithm, XMLStructure params) throws NoSuchAlgorithmException, InvalidAlgorithmParameterExceptionCreates aCanonicalizationMethodfor the specified algorithm URI and parameters. The parameters are specified as a mechanism-specificXMLStructure(ex:DOMStructure). This method is useful when the parameters are in XML form or there is no standard class for specifying the parameters.- Parameters:
algorithm- the URI identifying the canonicalization algorithmparams- a mechanism-specific XML structure from which to unmarshal the parameters from (may benullif not required or optional)- Returns:
- the
CanonicalizationMethod - Throws:
ClassCastException- if the type ofparamsis inappropriate for thisXMLSignatureFactoryInvalidAlgorithmParameterException- if the specified parameters are inappropriate for the requested algorithmNoSuchAlgorithmException- if an implementation of the specified algorithm cannot be foundNullPointerException- ifalgorithmisnull
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getKeyInfoFactory
Returns aKeyInfoFactorythat createsKeyInfoobjects. The returnedKeyInfoFactoryhas the same mechanism type and provider as thisXMLSignatureFactory.- Returns:
- a
KeyInfoFactory - Throws:
NoSuchMechanismException- if aKeyFactoryimplementation with the same mechanism type and provider is not available
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unmarshalXMLSignature
public abstract XMLSignature unmarshalXMLSignature(XMLValidateContext context) throws MarshalExceptionUnmarshals a newXMLSignatureinstance from a mechanism-specificXMLValidateContextinstance.- Parameters:
context- a mechanism-specific context from which to unmarshal the signature from- Returns:
- the
XMLSignature - Throws:
NullPointerException- ifcontextisnullClassCastException- if the type ofcontextis inappropriate for this factoryMarshalException- if an unrecoverable exception occurs during unmarshalling
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unmarshalXMLSignature
public abstract XMLSignature unmarshalXMLSignature(XMLStructure xmlStructure) throws MarshalExceptionUnmarshals a newXMLSignatureinstance from a mechanism-specificXMLStructureinstance. This method is useful if you only want to unmarshal (and not validate) anXMLSignature.- Parameters:
xmlStructure- a mechanism-specific XML structure from which to unmarshal the signature from- Returns:
- the
XMLSignature - Throws:
NullPointerException- ifxmlStructureisnullClassCastException- if the type ofxmlStructureis inappropriate for this factoryMarshalException- if an unrecoverable exception occurs during unmarshalling
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isFeatureSupported
Indicates whether a specified feature is supported.- Parameters:
feature- the feature name (as an absolute URI)- Returns:
trueif the specified feature is supported,falseotherwise- Throws:
NullPointerException- iffeatureisnull
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getURIDereferencer
Returns a reference to theURIDereferencerthat is used by default to dereference URIs inReferenceobjects.- Returns:
- a reference to the default
URIDereferencer(nevernull)
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