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public interface Instrumentation
This class provides services needed to instrument Java programming language code. Instrumentation is the addition of byte-codes to methods for the purpose of gathering data to be utilized by tools. Since the changes are purely additive, these tools do not modify application state or behavior. Examples of such benign tools include monitoring agents, profilers, coverage analyzers, and event loggers.
There are two ways to obtain an instance of the
Instrumentation
interface:
When a JVM is launched in a way that indicates an agent
class. In that case an Instrumentation
instance
is passed to the premain
method of the agent class.
When a JVM provides a mechanism to start agents sometime
after the JVM is launched. In that case an Instrumentation
instance is passed to the agentmain
method of the
agent code.
These mechanisms are described in the {@linkplain java.lang.instrument package specification}.
Once an agent acquires an Instrumentation
instance,
the agent may call methods on the instance at any time.
Method Summary | |
---|---|
void |
addTransformer(ClassFileTransformer transformer) Registers the supplied transformer. |
void |
addTransformer(ClassFileTransformer transformer, boolean canRetransform) Registers the supplied transformer. |
void |
Specifies a JAR file with instrumentation classes to be defined by the bootstrap class loader. |
void |
appendToSystemClassLoaderSearch(JarFile jarfile) Specifies a JAR file with instrumentation classes to be defined by the system class loader. |
Class[] |
Returns an array of all classes currently loaded by the JVM. |
Class[] |
getInitiatedClasses(ClassLoader loader) Returns an array of all classes for which loader is an initiating loader.
|
long |
getObjectSize(Object objectToSize) Returns an implementation-specific approximation of the amount of storage consumed by the specified object. |
boolean |
isModifiableClass(Class theClass) Determines whether a class is modifiable by java.lang.instrument.Instrumentation.retransformClasses or java.lang.instrument.Instrumentation.redefineClasses. |
boolean |
Returns whether the current JVM configuration supports java.lang.instrument.Instrumentation.setNativeMethodPrefix. |
boolean |
Returns whether or not the current JVM configuration supports redefinition of classes. |
boolean |
Returns whether or not the current JVM configuration supports retransformation of classes. |
void |
redefineClasses(ClassDefinition[] definitions) Redefine the supplied set of classes using the supplied class files. |
boolean |
removeTransformer(ClassFileTransformer transformer) Unregisters the supplied transformer. |
void |
retransformClasses(Class[] classes) Retransform the supplied set of classes. |
void |
setNativeMethodPrefix(ClassFileTransformer transformer, String prefix) This method modifies the failure handling of native method resolution by allowing retry with a prefix applied to the name. |
Method Detail |
---|
public void addTransformer(ClassFileTransformer transformer)
Same as addTransformer(transformer, false)
.
transformer
- the transformer to registerpublic void addTransformer(ClassFileTransformer transformer, boolean canRetransform)
canRetransform
is true,
when they are {@linkplain #retransformClasses retransformed}.
See {@link java.lang.instrument.ClassFileTransformer#transform
ClassFileTransformer.transform} for the order
of transform calls.
If a transformer throws
an exception during execution, the JVM will still call the other registered
transformers in order. The same transformer may be added more than once,
but it is strongly discouraged -- avoid this by creating a new instance of
tranformer class.
This method is intended for use in instrumentation, as described in the {@linkplain Instrumentation class specification}.
transformer
- the transformer to registercanRetransform
- can this transformer's transformations be retransformedpublic void appendToBootstrapClassLoaderSearch(JarFile jarfile)
When the virtual machine's built-in class loader, known as the "bootstrap class loader", unsuccessfully searches for a class, the entries in the {@link java.util.jar.JarFile JAR file} will be searched as well.
This method may be used multiple times to add multiple JAR files to be searched in the order that this method was invoked.
The agent should take care to ensure that the JAR does not contain any
classes or resources other than those to be defined by the bootstrap
class loader for the purpose of instrumentation.
Failure to observe this warning could result in unexpected
behaviour that is difficult to diagnose. For example, suppose there is a
loader L, and L's parent for delegation is the bootstrap class loader.
Furthermore, a method in class C, a class defined by L, makes reference to
a non-public accessor class C$1. If the JAR file contains a class C$1 then
the delegation to the bootstrap class loader will cause C$1 to be defined
by the bootstrap class loader. In this example an IllegalAccessError
will be thrown that may cause the application to fail. One approach to
avoiding these types of issues, is to use a unique package name for the
instrumentation classes.
The Java Virtual Machine Specification specifies that a subsequent attempt to resolve a symbolic reference that the Java virtual machine has previously unsuccessfully attempted to resolve always fails with the same error that was thrown as a result of the initial resolution attempt. Consequently, if the JAR file contains an entry that corresponds to a class for which the Java virtual machine has unsuccessfully attempted to resolve a reference, then subsequent attempts to resolve that reference will fail with the same error as the initial attempt.
jarfile
- The JAR file to be searched when the bootstrap class loader
unsuccessfully searches for a class.public void appendToSystemClassLoaderSearch(JarFile jarfile)
This method may be used multiple times to add multiple JAR files to be searched in the order that this method was invoked.
The agent should take care to ensure that the JAR does not contain any classes or resources other than those to be defined by the system class loader for the purpose of instrumentation. Failure to observe this warning could result in unexpected behaviour that is difficult to diagnose (see {@link #appendToBootstrapClassLoaderSearch appendToBootstrapClassLoaderSearch}.
The system class loader supports adding a JAR file to be searched if
it implements a method named appendToClassPathForInstrumentation
which takes a single parameter of type java.lang.String
. The
method is not required to have public
access. The name of
the JAR file is obtained by invoking the {@link java.util.zip.ZipFile#getName
getName()} method on the jarfile
and this is provided as the
parameter to the appendtoClassPathForInstrumentation
method.
The Java Virtual Machine Specification specifies that a subsequent attempt to resolve a symbolic reference that the Java virtual machine has previously unsuccessfully attempted to resolve always fails with the same error that was thrown as a result of the initial resolution attempt. Consequently, if the JAR file contains an entry that corresponds to a class for which the Java virtual machine has unsuccessfully attempted to resolve a reference, then subsequent attempts to resolve that reference will fail with the same error as the initial attempt.
This method does not change the value of java.class.path
{@link java.lang.System#getProperties system property}.
jarfile
- The JAR file to be searched when the system class loader
unsuccessfully searches for a class.public Class[] getAllLoadedClasses()
public Class[] getInitiatedClasses(ClassLoader loader)
loader
is an initiating loader.
If the supplied loader is null
, classes initiated by the bootstrap class
loader are returned.
loader
- the loader whose initiated class list will be returnedpublic long getObjectSize(Object objectToSize)
objectToSize
- the object to sizepublic boolean isModifiableClass(Class theClass)
true
.
If a class is not modifiable then this method returns false
.
For a class to be retransformed, {@link #isRetransformClassesSupported} must also be true.
But the value of isRetransformClassesSupported()
does not influence the value
returned by this function.
For a class to be redefined, {@link #isRedefineClassesSupported} must also be true.
But the value of isRedefineClassesSupported()
does not influence the value
returned by this function.
Primitive classes (for example, java.lang.Integer.TYPE
)
and array classes are never modifiable.
theClass
public boolean isNativeMethodPrefixSupported()
Can-Set-Native-Method-Prefix
manifest attribute is set to
true
in the agent JAR file (as described in the
{@linkplain java.lang.instrument package specification}) and the JVM supports
this capability.
During a single instantiation of a single JVM, multiple
calls to this method will always return the same answer.
public boolean isRedefineClassesSupported()
Can-Redefine-Classes
manifest attribute is set to
true
in the agent JAR file (as described in the
{@linkplain java.lang.instrument package specification}) and the JVM supports
this capability.
During a single instantiation of a single JVM, multiple calls to this
method will always return the same answer.
public boolean isRetransformClassesSupported()
Can-Retransform-Classes
manifest attribute is set to
true
in the agent JAR file (as described in the
{@linkplain java.lang.instrument package specification}) and the JVM supports
this capability.
During a single instantiation of a single JVM, multiple calls to this
method will always return the same answer.
public void redefineClasses(ClassDefinition[] definitions) throws ClassNotFoundException, UnmodifiableClassException
This method is used to replace the definition of a class without reference to the existing class file bytes, as one might do when recompiling from source for fix-and-continue debugging. Where the existing class file bytes are to be transformed (for example in bytecode instrumentation) {@link #retransformClasses retransformClasses} should be used.
This method operates on a set in order to allow interdependent changes to more than one class at the same time (a redefinition of class A can require a redefinition of class B).
If a redefined method has active stack frames, those active frames continue to run the bytecodes of the original method. The redefined method will be used on new invokes.
This method does not cause any initialization except that which would occur under the customary JVM semantics. In other words, redefining a class does not cause its initializers to be run. The values of static variables will remain as they were prior to the call.
Instances of the redefined class are not affected.
The redefinition may change method bodies, the constant pool and attributes. The redefinition must not add, remove or rename fields or methods, change the signatures of methods, or change inheritance. These restrictions maybe be lifted in future versions. The class file bytes are not checked, verified and installed until after the transformations have been applied, if the resultant bytes are in error this method will throw an exception.
If this method throws an exception, no classes have been redefined.
This method is intended for use in instrumentation, as described in the {@linkplain Instrumentation class specification}.
definitions
- array of classes to redefine with corresponding definitions;
a zero-length array is allowed, in this case, this method does nothingClassNotFoundException
- Can never be thrown (present for compatibility reasons only)UnmodifiableClassException
- if a specified class cannot be modified
({@link #isModifiableClass} would return false
)public boolean removeTransformer(ClassFileTransformer transformer)
transformer
- the transformer to unregisterpublic void retransformClasses(Class[] classes) throws UnmodifiableClassException
This function facilitates the instrumentation of already loaded classes. When classes are initially loaded or when they are {@linkplain #redefineClasses redefined}, the initial class file bytes can be transformed with the {@link java.lang.instrument.ClassFileTransformer ClassFileTransformer}. This function reruns the transformation process (whether or not a transformation has previously occurred). This retransformation follows these steps:
canRetransform
false, the bytes returned by
{@link java.lang.instrument.ClassFileTransformer#transform transform}
during the last class load or redefine are
reused as the output of the transformation; note that this is
equivalent to reapplying the previous transformation, unaltered;
except that
{@link java.lang.instrument.ClassFileTransformer#transform transform}
is not called
canRetransform
true, the
{@link java.lang.instrument.ClassFileTransformer#transform transform}
method is called in these transformers
The order of transformation is described in the ({@link java.lang.instrument.ClassFileTransformer#transform transform} method. This same order is used in the automatic reapplication of retransformation incapable transforms.
The initial class file bytes represent the bytes passed to {@link java.lang.ClassLoader#defineClass ClassLoader.defineClass} or {@link #redefineClasses redefineClasses} (before any transformations were applied), however they might not exactly match them. The constant pool might not have the same layout or contents. The constant pool may have more or fewer entries. Constant pool entries may be in a different order; however, constant pool indices in the bytecodes of methods will correspond. Some attributes may not be present. Where order is not meaningful, for example the order of methods, order might not be preserved.
This method operates on a set in order to allow interdependent changes to more than one class at the same time (a retransformation of class A can require a retransformation of class B).
If a retransformed method has active stack frames, those active frames continue to run the bytecodes of the original method. The retransformed method will be used on new invokes.
This method does not cause any initialization except that which would occur under the customary JVM semantics. In other words, redefining a class does not cause its initializers to be run. The values of static variables will remain as they were prior to the call.
Instances of the retransformed class are not affected.
The retransformation may change method bodies, the constant pool and attributes. The retransformation must not add, remove or rename fields or methods, change the signatures of methods, or change inheritance. These restrictions maybe be lifted in future versions. The class file bytes are not checked, verified and installed until after the transformations have been applied, if the resultant bytes are in error this method will throw an exception.
If this method throws an exception, no classes have been retransformed.
This method is intended for use in instrumentation, as described in the {@linkplain Instrumentation class specification}.
classes
- array of classes to retransform;
a zero-length array is allowed, in this case, this method does nothingUnmodifiableClassException
- if a specified class cannot be modified
({@link #isModifiableClass} would return false
)public void setNativeMethodPrefix(ClassFileTransformer transformer, String prefix)
native boolean foo(int x);We could transform the class file (with the ClassFileTransformer during the initial definition of the class) so that this becomes:
boolean foo(int x) { ... record entry to foo ... return wrapped_foo(x); } native boolean wrapped_foo(int x);Where
foo
becomes a wrapper for the actual native
method with the appended prefix "wrapped_". Note that
"wrapped_" would be a poor choice of prefix since it
might conceivably form the name of an existing method
thus something like "$$$MyAgentWrapped$$$_" would be
better but would make these examples less readable.
The wrapper will allow data to be collected on the native
method call, but now the problem becomes linking up the
wrapped method with the native implementation.
That is, the method wrapped_foo
needs to be
resolved to the native implementation of foo
,
which might be:
Java_somePackage_someClass_foo(JNIEnv* env, jint x)This function allows the prefix to be specified and the proper resolution to occur. Specifically, when the standard resolution fails, the resolution is retried taking the prefix into consideration. There are two ways that resolution occurs, explicit resolution with the JNI function
RegisterNatives
and the normal automatic resolution. For
RegisterNatives
, the JVM will attempt this
association:
method(foo) -> nativeImplementation(foo)When this fails, the resolution will be retried with the specified prefix prepended to the method name, yielding the correct resolution:
method(wrapped_foo) -> nativeImplementation(foo)For automatic resolution, the JVM will attempt:
method(wrapped_foo) -> nativeImplementation(wrapped_foo)When this fails, the resolution will be retried with the specified prefix deleted from the implementation name, yielding the correct resolution:
method(wrapped_foo) -> nativeImplementation(foo)Note that since the prefix is only used when standard resolution fails, native methods can be wrapped selectively. Since each
ClassFileTransformer
can do its own transformation of the bytecodes, more
than one layer of wrappers may be applied. Thus each
transformer needs its own prefix. Since transformations
are applied in order, the prefixes, if applied, will
be applied in the same order
(see {@link #addTransformer(ClassFileTransformer,boolean) addTransformer}).
Thus if three transformers applied
wrappers, foo
might become
$trans3_$trans2_$trans1_foo
. But if, say,
the second transformer did not apply a wrapper to
foo
it would be just
$trans3_$trans1_foo
. To be able to
efficiently determine the sequence of prefixes,
an intermediate prefix is only applied if its non-native
wrapper exists. Thus, in the last example, even though
$trans1_foo
is not a native method, the
$trans1_
prefix is applied since
$trans1_foo
exists.
transformer
- The ClassFileTransformer which wraps using this prefix.prefix
- The prefix which has been applied to wrapped native methods.
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