Object to the caller, then this object will share no common superclass with any other object! The security of the system can be compromised if your class loader returned its own value of java. SecurityManager , which did not have the same checks as the real one did. After the initial checks, we come to the code above which is where the simple class loader gets an opportunity to load an implementation of this class.
I chose this technique in the example so that there would be no question of the primordial class loader finding our class. Note that the sun. If the class implementation was loaded, the penultimate step is to call the defineClass method from java. ClassLoader , which can be considered the first step of class verification. This method is implemented in the Java virtual machine and is responsible for verifying that the class bytes are a legal Java class file.
Internally, the defineClass method fills out a data structure that the JVM uses to hold classes. If the class data is malformed, this call will cause a ClassFormatError to be thrown. The last class loader-specific requirement is to call resolveClass if the boolean parameter resolveIt was true.
This method does two things: First, it causes any classes that are referenced by this class explicitly to be loaded and a prototype object for this class to be created; then, it invokes the verifier to do dynamic verification of the legitimacy of the bytecodes in this class.
If verification fails, this method call will throw a LinkageError , the most common of which is a VerifyError.
Note that for any class you will load, the resolveIt variable will always be true. It is only when the system is recursively calling loadClass that it may set this variable false because it knows the class it is asking for is already resolved.
The final step in the process is to store the class we've loaded and resolved into our hash table so that we can return it again if need be, and then to return the Class reference to the caller. Of course if it were this simple there wouldn't be much more to talk about. In fact, there are two issues that class loader builders will have to deal with, security and talking to classes loaded by the custom class loader. Whenever you have an application loading arbitrary classes into the system through your class loader, your application's integrity is at risk.
This is due to the power of the class loader. Let's take a moment to look at one of the ways a potential villain could break into your application if you aren't careful. In our simple class loader, if the primordial class loader couldn't find the class, we loaded it from our private repository.
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How does the hashCode method of java works? Class loaders that support concurrent loading of classes are known as parallel capable class loaders and are required to register themselves at their class initialization time by invoking the ClassLoader. Note that the ClassLoader class is registered as parallel capable by default. However, its subclasses still need to register themselves if they are parallel capable. In environments in which the delegation model is not strictly hierarchical, class loaders need to be parallel capable, otherwise class loading can lead to deadlocks because the loader lock is held for the duration of the class loading process see loadClass methods.
Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. However, some classes may not originate from a file; they may originate from other sources, such as the network, or they could be constructed by an application.
The method defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using Class. The methods and constructors of objects created by a class loader may reference other classes.
To determine the class es referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class. For example, an application could create a network class loader to download class files from a server. The network class loader subclass must define the methods findClass and loadClassData to load a class from the network.
Once it has downloaded the bytes that make up the class, it should use the method defineClass to create a class instance. Examples of valid class names include: "java. String" "javax. Method Summary Methods Modifier and Type Method and Description void clearAssertionStatus Sets the default assertion status for this class loader to false and discards any package defaults or class assertion status settings associated with the class loader.
ClassLoader getParent Returns the parent class loader for delegation. Methods inherited from class java. Object clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait Constructor Detail ClassLoader protected ClassLoader ClassLoader parent Creates a new class loader using the specified parent class loader for delegation.
If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception. Invoke findLoadedClass String to check if the class has already been loaded. Invoke the loadClass method on the parent class loader.
If the parent is null the class loader built-in to the virtual machine is used, instead. Invoke the findClass String method to find the class. If the class was found using the above steps, and the resolve flag is true, this method will then invoke the resolveClass Class method on the resulting Class object. Subclasses of ClassLoader are encouraged to override findClass String , rather than this method.
Unless overridden, this method synchronizes on the result of getClassLoadingLock method during the entire class loading process. For backward compatibility, the default implementation of this method behaves as follows. If this ClassLoader object is registered as parallel capable, the method returns a dedicated object associated with the specified class name.
Otherwise, the method returns this ClassLoader object. This method assigns a default ProtectionDomain to the newly defined class. The ProtectionDomain is effectively granted the same set of permissions returned when Policy. The default domain is created on the first invocation of defineClass , and re-used on subsequent invocations. To assign a specific ProtectionDomain to the class, use the defineClass method that takes a ProtectionDomain as one of its arguments.
The first class defined in a package determines the exact set of certificates that all subsequent classes defined in that package must contain. The set of certificates for a class is obtained from the CodeSource within the ProtectionDomain of the class.
Any classes added to that package must contain the same set of certificates or a SecurityException will be thrown. Question I don't understand why Custom would try to load com. Improve this question. Add a comment. Active Oldest Votes. Small intro As you already know, by default Java uses the bootstrap classloader and the system classloader. The answer The class com. A bit more about each option: At application start-up : you can define when starting a JVM instance that instead of using Java's default system class loader you want to use your own.
To do so, simply call java with the following environment variable defined: -Djava. Custom In this case what happens is that all classes from your application in that JVM instance will actually be loaded by Custom class loader.
SomeClass" ; Like Noofiz said in his answer once you have one class loaded all referenced classes that are required and not yet loaded are loaded through the associated class loader.
Some extra info Usually the best way to implement a custom class loader is to use the delegation model as you mentioned. Improve this answer. Mikhail Mikhail 4, 12 12 silver badges 30 30 bronze badges.
How can any custom classloader be used then? If I start from the some class loaded by Bootstrap , then every class would have to use the Bootstrap classloader because it would be called by the class loaded in bootstrap.
I would therefore never move to System or Custom. Then somewhere you create your own class loader and instantiate some class through it. And all class instantiated through this class, would have your class loader as current. Which in tern will have a Bootstrap as a parent.
Noofiz I feel that mentioning that you can instantiate a classloader and then use it to instantiate other classes would significantly improve the answer. Would you care to edit it to include additional explanations or examples? Because the classloader delegation system specifies that the standard way to load classes is delegate 1st to the classloader's parent and only on fail try to load the class.
That means that even though you use the method's class classloader, it still might be it's parent loading the class.
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