有没有可能知道一个Java类是否被加载,而不需要试图加载它? Class.forName
会尝试加载该类,但我不想产生这种副作用。还有其他方法吗?
(我不想覆盖类加载器。我正在寻找一种相对简单的方法。)
有没有可能知道一个Java类是否被加载,而不需要试图加载它? Class.forName
会尝试加载该类,但我不想产生这种副作用。还有其他方法吗?
(我不想覆盖类加载器。我正在寻找一种相对简单的方法。)
(感谢Aleksi)此代码:
public class TestLoaded {
public static void main(String[] args) throws Exception {
java.lang.reflect.Method m = ClassLoader.class.getDeclaredMethod("findLoadedClass", new Class[] { String.class });
m.setAccessible(true);
ClassLoader cl = ClassLoader.getSystemClassLoader();
Object test1 = m.invoke(cl, "TestLoaded$ClassToTest");
System.out.println(test1 != null);
ClassToTest.reportLoaded();
Object test2 = m.invoke(cl, "TestLoaded$ClassToTest");
System.out.println(test2 != null);
}
static class ClassToTest {
static {
System.out.println("Loading " + ClassToTest.class.getName());
}
static void reportLoaded() {
System.out.println("Loaded");
}
}
}
生成:
请注意,示例类不在包中。需要完整的二进制名称。false Loading TestLoaded$ClassToTest Loaded true
"java.security.KeyStore$Builder$FileBuilder$1"
。// 获取 ClassLoader 类的 findLoadedClass(String) 方法
Method m = ClassLoader.class.getDeclaredMethod("findLoadedClass", new Class[]{String.class});
// 设置该方法可访问
m.setAccessible(true);
// 调用该方法,查找名为 "String" 的类
m.invoke(t, "String");
- Elbek一种实现此目的的方法是使用instrumentation API编写Java代理。这将允许您记录JVM加载类的过程。
public class ClassLoadedAgent implements ClassFileTransformer {
private static ClassLoadedAgent AGENT = null;
/** Agent "main" equivalent */
public static void premain(String agentArguments,
Instrumentation instrumentation) {
AGENT = new ClassLoadedAgent();
for (Class<?> clazz : instrumentation.getAllLoadedClasses()) {
AGENT.add(clazz);
}
instrumentation.addTransformer(AGENT);
}
private final Map<ClassLoader, Set<String>> classMap = new WeakHashMap<ClassLoader, Set<String>>();
private void add(Class<?> clazz) {
add(clazz.getClassLoader(), clazz.getName());
}
private void add(ClassLoader loader, String className) {
synchronized (classMap) {
System.out.println("loaded: " + className);
Set<String> set = classMap.get(loader);
if (set == null) {
set = new HashSet<String>();
classMap.put(loader, set);
}
set.add(className);
}
}
private boolean isLoaded(String className, ClassLoader loader) {
synchronized (classMap) {
Set<String> set = classMap.get(loader);
if (set == null) {
return false;
}
return set.contains(className);
}
}
@Override
public byte[] transform(ClassLoader loader, String className,
Class<?> classBeingRedefined, ProtectionDomain protectionDomain,
byte[] classfileBuffer) throws IllegalClassFormatException {
add(loader, className);
return classfileBuffer;
}
public static boolean isClassLoaded(String className, ClassLoader loader) {
if (AGENT == null) {
throw new IllegalStateException("Agent not initialized");
}
if (loader == null || className == null) {
throw new IllegalArgumentException();
}
while (loader != null) {
if (AGENT.isLoaded(className, loader)) {
return true;
}
loader = loader.getParent();
}
return false;
}
}
META-INF/MANIFEST.MF:
Manifest-Version: 1.0
Premain-Class: myinstrument.ClassLoadedAgent
java -javaagent:myagent.jar ....etcetera
myinstrument
是什么意思吗? - user489041public class TestLoaded {
public static boolean loaded = false;
public static void main(String[] args) throws ClassNotFoundException {
System.out.println(loaded);
ClassToTest.reportLoaded();
System.out.println(loaded);
}
static class ClassToTest {
static {
System.out.println("Loading");
TestLoaded.loaded = true;
}
static void reportLoaded() {
System.out.println("Loaded");
}
}
}
输出:
false Loading Loaded true
Object o = ClassToTest.class;
时,类会被加载但不会被初始化。我猜,提问者并不在意这个细节;只是想搞清楚而已。 - maaartinusInputStream is = getResourceAsStream(name);
name
是类文件的路径,例如 com/blah/blah/blah/foo.class
。
当类没有被加载到我的类加载器或系统类加载器中时,getResourceAsStream
返回 null
,当类已经被加载时返回非空值。
/
)。它不会检查该类是否由当前类加载器加载。 - Lukas Eder