8293174: Handle parallel class loading in Java

[coleenp]

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Issue

• JDK-8293174: Handle parallel class loading in Java

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[coleenp]

So I decided that the synchronized/parallel loadClassInternal logic shouldn't be in ClassLoader.java class which is a basic class and we're still investigating whether there's anything in the spec that mandates this synchronization, or if it's longstanding behavior that we've had. We may only want to support it for a limited time for compatibility. I moved the code to jdk.internal.loader.SynchronizedLoader. Having a single hashtable for all class loaders is more space efficient, and still mimics the JVM behavior because in the VM we used a single global PlaceholderTable, so not worse for performance.

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