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java - 在 Groovy 中重新加载类

问题描述

我有一个自定义的 ClassLoader 扩展GroovyClassLoader，它将源代码编译为.class磁盘上的文件，然后加载生成的类：

class MyClassLoader extends GroovyClassLoader {

  File cache = new File( './cache' )
  Compiler compiler

  MyClassLoader() {
    CompilerConfiguration cc = new CompilerConfiguration( targetDirectory:cache )
    compiler = new Compiler( cc )
    addClasspath cache.path
  }

  @Override
  Class findClass( name ) {
    try{
      parent.findClass name
    }catch( ClassNotFoundException e ){
      compiler.compile name, getBodySomehow()
      byte[] blob = loadFromFileSystem name
      Class c = defineClass name, blob, 0, blob.length
      setClassCacheEntry c
      c
    }
  }

  @Override
  void removeClassCacheEntry​(String name) {
    Class c = cache[ name ]
    super.removeClassCacheEntry​(name)
    GroovySystem.metaClassRegistry.removeMetaClass c
    deleteFiles name
  }
}

Class clazz = myClassLoader.loadClass 'some.pckg.SomeClass'

现在，如果我更改源代码，请致电myClassLoader.removeClassCacheEntry​(name)并重试myClassLoader.loadClass()，我得到：

java.lang.LinkageError：加载程序（com/my/MyClassLoader 的实例）：尝试为名称 some/pckg/SomeClass 重复类定义

我阅读了互联网的大半部分，并找到了一个“解决方案”来为每个类初始化一个类加载器：

MyClassLoader myClassLoader = new MyClassLoader()
Class clazz = myClassLoader.loadClass 'some.pckg.SomeClass'

这似乎有效，但引起了我的性能问题......

重新加载类的正确方法是什么？如何重用相同的类加载器？我错过了什么？

标签： javaclassgroovy

解决方案

其实有一个技巧可以使用

原来，当你打电话

classLoader.defineClass(className, classBytes, 0, classBytes.length)

defineClass1它调用实​​际调用方法的java本机loadClass方法。

因此，可以拦截此方法并对其进行处理，使其与原始方法有所不同。

在包含缓存的类文件的文件夹中，我将以下 groovy 编译为类：A.class

println "Hello World!"

B.class检查依赖类加载

class B extends A {
    def run(){
        super.run()
        println "Hello from ${this.getClass()}!"
    }
}

并C.class检查多级类加载

我用这个jar编译下面的类并运行类重新加载示例

class C extends org.apache.commons.lang3.RandomUtils {
    def rnd(){ nextInt() }
}

以下类+代码加载并重新加载同一个类：

import java.security.PrivilegedAction;
import java.security.AccessController;
import org.codehaus.groovy.control.CompilationFailedException;

@groovy.transform.CompileStatic
class CacheClassLoader extends GroovyClassLoader{
    private File cacheDir = new File('/11/tmp/a__cache')

    private CacheClassLoader(){throw new RuntimeException("default constructor not allowed")}

    public CacheClassLoader(ClassLoader parent){
        super(parent)
    }
    public CacheClassLoader(Script parent){
        this(parent.getClass().getClassLoader())
    }

    @Override
    protected Class getClassCacheEntry(String name) {
        Class clazz = super.getClassCacheEntry(name)
        if( clazz ){
            println "getClassCacheEntry $name -> got from memory cache"
            return clazz
        }
        def cacheFile = new File(cacheDir, name.tr('.','/')+'.class')
        if( cacheFile.exists() ){
            println "getClassCacheEntry $name -> cache file exists, try to load it"
            //clazz = getPrivelegedLoader().defineClass(name, cacheFile.bytes)
            clazz = getPrivelegedLoader().defineClass(name, cacheFile.bytes)
            super.setClassCacheEntry(clazz)
        }
        return clazz
    }

    private PrivelegedLoader getPrivelegedLoader(){
        PrivelegedLoader loader = AccessController.doPrivileged(new PrivilegedAction<PrivelegedLoader>() {
            public PrivelegedLoader run() {
                return new PrivelegedLoader();
            }
        });
    }
    public class PrivelegedLoader extends CacheClassLoader {
        private final CacheClassLoader delegate

        public PrivelegedLoader(){ 
            super(CacheClassLoader.this)
            this.delegate = CacheClassLoader.this
        }

        public Class loadClass(String name, boolean lookupScriptFiles, boolean preferClassOverScript, boolean resolve) throws ClassNotFoundException, CompilationFailedException {
            Class c = findLoadedClass(name);
            if (c != null) return c;
            return delegate.loadClass(name, lookupScriptFiles, preferClassOverScript, resolve);
        }
    }
}

def c=null
//just to show intermediate class loaders could load some classes that will be used in CacheClassLoader
def cl_0 = new GroovyClassLoader(this.getClass().getClassLoader())
cl_0.addClasspath('/11/tmp/a__cache/commons-lang3-3.5.jar')
//create cache class loader
def cl = new CacheClassLoader(cl_0)

println "---1---"
c = cl.loadClass('A')
c.newInstance().run()

println "---2---"
c = cl.loadClass('A')
c.newInstance().run()

println "---3---"
cl.removeClassCacheEntry('A')
c = cl.loadClass('A')
c.newInstance().run()

println "---4---"
c = cl.loadClass('B')
c.newInstance().run()

println "---5---"
cl.removeClassCacheEntry('A')
cl.removeClassCacheEntry('B')
c = cl.loadClass('B')
c.newInstance().run()

println "---6---"
c = cl.loadClass('C')
println c.newInstance().rnd()

结果：

---1---
getClassCacheEntry A -> cache file exists, try to load it
Hello World!
---2---
getClassCacheEntry A -> got from memory cache
Hello World!
---3---
getClassCacheEntry A -> cache file exists, try to load it
Hello World!
---4---
getClassCacheEntry B -> cache file exists, try to load it
getClassCacheEntry A -> got from memory cache
Hello World!
Hello from class B!
---5---
getClassCacheEntry B -> cache file exists, try to load it
getClassCacheEntry A -> cache file exists, try to load it
Hello World!
Hello from class B!
---6---
getClassCacheEntry C -> cache file exists, try to load it
226399895

PS：不确定是否需要特权访问

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