Implementation of hotspot reference

One .referencePolicy

share.vm.memory.referencePolicy Is used for java.lang.SoftReference Instance cleanup .should_clear_reference Method parameters p Refer to SoftReference object .

class ReferencePolicy : public CHeapObj<mtGC> {
 public:
  virtual bool should_clear_reference(oop p, jlong timestamp_clock) {
    ShouldNotReachHere();
    return true;
  }

  // Capture state (of-the-VM) information needed to evaluate the policy
  virtual void setup() { /* do nothing */ }
};

should_clear_reference Method to determine whether a soft reference object needs to be cleaned up ,ReferencePolicy Parent class , There are four subclasses , Namely

class NeverClearPolicy : public ReferencePolicy {
 public:
  virtual bool should_clear_reference(oop p, jlong timestamp_clock) {
    return false;
  }
};

class AlwaysClearPolicy : public ReferencePolicy {
 public:
  virtual bool should_clear_reference(oop p, jlong timestamp_clock) {
    return true;
  }
};

class LRUCurrentHeapPolicy : public ReferencePolicy {
 private:
  jlong _max_interval;

 public:
  LRUCurrentHeapPolicy();
  void setup();
  virtual bool should_clear_reference(oop p, jlong timestamp_clock);
};

class LRUMaxHeapPolicy : public ReferencePolicy {
 private:
  jlong _max_interval;

 public:
  LRUMaxHeapPolicy();

  // Capture state (of-the-VM) information needed to evaluate the policy
  void setup();
  virtual bool should_clear_reference(oop p, jlong timestamp_clock);
};

NeverClearPolicy should_clear_reference Method directly returns false,AlwaysClearPolicy should_clear_reference Method directly returns true.
The main explanation is LRUCurrentHeapPolicy and LRUMaxHeapPolicy.LRUMaxHeapPolicy Using the largest amount of free space MB Count ,LRUCurrentHeapPolicy Using the free space of the current heap MB Count .

LRUCurrentHeapPolicy::LRUCurrentHeapPolicy() {
  setup();
}

// Capture state (of-the-VM) information needed to evaluate the policy
void LRUCurrentHeapPolicy::setup() {
// The current heap is the largest free MB Count ,SoftRefLRUPolicyMSPerMB each 1M Free space can be maintained SoftReference The length of time an object will survive , The default value is 1000
  _max_interval = (Universe::get_heap_free_at_last_gc() / M) * SoftRefLRUPolicyMSPerMB;
  assert(_max_interval >= 0,"Sanity check");
}

// The oop passed in is the SoftReference object, and not
// the object the SoftReference points to.
bool LRUCurrentHeapPolicy::should_clear_reference(oop p,
                                                  jlong timestamp_clock) {
  //timestamp Method returns SoftReference Class timestamp attribute , This property is called every time get Method to get the soft reference object will be updated to a clock                                                       
  jlong interval = timestamp_clock - java_lang_ref_SoftReference::timestamp(p);
  assert(interval >= 0, "Sanity check");

  // The interval will be zero if the ref was accessed since the last scavenge/gc.
  if(interval <= _max_interval) {
    return false;
  }

  return true;
}

/// MaxHeap //

LRUMaxHeapPolicy::LRUMaxHeapPolicy() {
  setup();
}

// Capture state (of-the-VM) information needed to evaluate the policy
void LRUMaxHeapPolicy::setup() {
  size_t max_heap = MaxHeapSize;
  // Of the current remaining free space MB Count 
  max_heap -= Universe::get_heap_used_at_last_gc();
  max_heap /= M;

  _max_interval = max_heap * SoftRefLRUPolicyMSPerMB;
  assert(_max_interval >= 0,"Sanity check");
}

// The oop passed in is the SoftReference object, and not
// the object the SoftReference points to.
bool LRUMaxHeapPolicy::should_clear_reference(oop p,
                                             jlong timestamp_clock) {
   //timestamp Method returns SoftReference Class timestamp attribute , This property is called every time get Method to get the soft reference object will be updated to a clock      
  jlong interval = timestamp_clock - java_lang_ref_SoftReference::timestamp(p);
  assert(interval >= 0, "Sanity check");

  // The interval will be zero if the ref was accessed since the last scavenge/gc.
  if(interval <= _max_interval) {
    return false;
  }

  return true;
}

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2.referenceProcessor

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reference processor ,referenceProcessor.hpp in DiscoveredList Is stored Reference The linked list of ,DiscoveredListIterator Is traversal DiscoveredList The iterator .
referenceProcessor.cpp in process_discovered_references The method is actually used to handle the different types of DiscoveredList Medium Reference example , The underlying actual call process_discovered_reflist( Handling strong references ) and process_phaseJNI Method ( Deal with weak quotes ), Respectively in gc After the call , Here's the picture .

Specific methods are not analyzed , Because I won't .