Some thoughts on Java Record - the use of default methods and the underlying implementation of generating relevant bytecodes based on pre-compilation

Get started with the Record class quickly

Let's start with a simple example, declaring a user Record.

public record User(long id, String name, int age) {}

After writing the code like this, the elements and method implementations included by default in the Record class include:

1. The constituent elements ( int id, String name, int age) specified by the record header, and these elements are final.
2. record has only one constructor by default, which is the constructor that contains all elements.
3. Each element of the record has a corresponding getter (but this getter is not getxxx(), but is named directly with the variable name, so when using the serialization framework, the DAO framework must pay attention to this)
4. Implement good hashCode(), equals(), toString() methods (implemented by automatically generating bytecodes about hashCode(), equals(), toString() method implementations at compile stage).

Let's use this Record:

User zhx = new User(1, "zhx", 29);
User ttj = new User(2, "ttj", 25);
System.out.println(zhx.id());//1
System.out.println(zhx.name());//zhx
System.out.println(zhx.age());//29
System.out.println(zhx.equals(ttj));//false
System.out.println(zhx.toString());//User[id=1, name=zhx, age=29]
System.out.println(zhx.hashCode());//3739156

How the structure of Record is implemented

Bytecode inserted into relevant fields and methods after compilation

There are two ways to view the bytecode of the above example. One is to view the text version of the bytecode through the javap -v User.classcommand , and intercept the important bytecode as follows:

//省略文件头，文件常量池部分
{
  //public 构造器，全部属性作为参数，并给每个 Field 赋值
  public com.github.hashzhang.basetest.User(long, java.lang.String, int);
    descriptor: (JLjava/lang/String;I)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=5, args_size=4
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Record."<init>":()V
         4: aload_0
         5: lload_1
         6: putfield      #7                  // Field id:J
         9: aload_0
        10: aload_3
        11: putfield      #13                 // Field name:Ljava/lang/String;
        14: aload_0
        15: iload         4
        17: putfield      #17                 // Field age:I
        20: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      21     0  this   Lcom/github/hashzhang/basetest/User;
            0      21     1    id   J
            0      21     3  name   Ljava/lang/String;
            0      21     4   age   I
    MethodParameters:
      Name                           Flags
      id
      name
      age

  //public final 修饰的 toString 方法
  public final java.lang.String toString();
    descriptor: ()Ljava/lang/String;
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         //核心实现是这个 invokedynamic，我们后面会分析
         1: invokedynamic #21,  0             // InvokeDynamic #0:toString:(Lcom/github/hashzhang/basetest/User;)Ljava/lang/String;
         6: areturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/github/hashzhang/basetest/User;
  //public final 修饰的 hashCode 方法
  public final int hashCode();
    descriptor: ()I
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         //核心实现是这个 invokedynamic，我们后面会分析
         1: invokedynamic #25,  0             // InvokeDynamic #0:hashCode:(Lcom/github/hashzhang/basetest/User;)I
         6: ireturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/github/hashzhang/basetest/User;
  //public final 修饰的 equals 方法
  public final boolean equals(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Z
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         //核心实现是这个 invokedynamic，我们后面会分析
         2: invokedynamic #29,  0             // InvokeDynamic #0:equals:(Lcom/github/hashzhang/basetest/User;Ljava/lang/Object;)Z
         7: ireturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       8     0  this   Lcom/github/hashzhang/basetest/User;
            0       8     1     o   Ljava/lang/Object;
  //public 修饰的 id 的 getter
  public long id();
    descriptor: ()J
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: getfield      #7                  // Field id:J
         4: lreturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/github/hashzhang/basetest/User;
  //public 修饰的 name 的 getter
  public java.lang.String name();
    descriptor: ()Ljava/lang/String;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #13                 // Field name:Ljava/lang/String;
         4: areturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/github/hashzhang/basetest/User;
  //public 修饰的 age 的 getter
  public int age();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #17                 // Field age:I
         4: ireturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/github/hashzhang/basetest/User;
}
SourceFile: "User.java"
Record:
  long id;
    descriptor: J

  java.lang.String name;
    descriptor: Ljava/lang/String;

  int age;
    descriptor: I

//以下是 invokedynamic 会调用的方法以及参数信息，我们后面会分析
BootstrapMethods:
  0: #50 REF_invokeStatic java/lang/runtime/ObjectMethods.bootstrap:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/TypeDescriptor;Ljava/lang/Class;Ljava/lang/String;[Ljava/lang/invoke/MethodHandle;)Ljava
/lang/Object;
    Method arguments:
      #8 com/github/hashzhang/basetest/User
      #57 id;name;age
      #59 REF_getField com/github/hashzhang/basetest/User.id:J
      #60 REF_getField com/github/hashzhang/basetest/User.name:Ljava/lang/String;
      #61 REF_getField com/github/hashzhang/basetest/User.age:I
InnerClasses:
  public static final #67= #63 of #65;    // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles

The other is to view through the IDE's jclasslib plug-in. I recommend using this method. The viewing effect is as follows:

Automatically generated private final field

Auto-generated full-property constructor

Auto-generated public getter methods

Automatically generated hashCode(), equals(), toString() methods

At the heart of these methods is invokedynamic :

It seems to be calling another method . Is there no performance loss problem with this indirect call ? JVM developers have figured this out. Let's first understand invokedynamic .

Background generated by invokedynamic

Java was originally a statically typed language, which means that the main process of its type checking is mainly at compile time rather than run time. In order to be compatible with dynamic type syntax, and also for JVM to be compatible with dynamic languages ​​(the original intention of JVM is not to only run Java), the bytecode instruction invokedynamic was introduced in Java 7 . This is also the basis for the implementation of later Java 8's lambda expressions and var syntax.

invokedynamic 与 MethodHandle

invokedynamic is inseparable from the use of the java.lang.invoke package. The main purpose of this package is to provide a new mechanism for dynamically determining the target method, called MethodHandle.

MethodHandleYou can dynamically obtain the method you want to call for invocation, which Reflectionis similar to Java reflection, but in order to pursue performance efficiency, it needs to be used . The MethodHandlemain reason is: Reflectionit is only the Java language that supplements the implementation of reflection, and does not consider the issue of efficiency, especially JIT There is basically no efficient optimization for such reflection calls . MethodHandleIt is more like the simulation of method instruction invocation in bytecode. If used properly, JIT can also optimize it, for example, declaring MethodHandlerelated method references as static final:

private static final MutableCallSite callSite = new MutableCallSite(
        MethodType.methodType(int.class, int.class, int.class));
private static final MethodHandle invoker = callSite.dynamicInvoker();

Automatically generated toString(), hashcode(), equals() implementations

It can be seen from the bytecode that incokedynamic actually calls BoostrapMethodsthe #0 method in :

0 aload_0
1 invokedynamic #24 <hashCode, BootstrapMethods #0>
6 ireturn

The Bootstap method table includes:

BootstrapMethods:
  //调用的实际是 java.lang.runtime.ObjectMethods 的 boostrap 方法
  0: #50 REF_invokeStatic java/lang/runtime/ObjectMethods.bootstrap:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/TypeDescriptor;Ljava/lang/Class;Ljava/lang/String;[Ljava/lang/invoke/MethodHandle;)Ljava
/lang/Object;
    Method arguments:
      #8 com/github/hashzhang/basetest/User
      #57 id;name;age
      #59 REF_getField com/github/hashzhang/basetest/User.id:J
      #60 REF_getField com/github/hashzhang/basetest/User.name:Ljava/lang/String;
      #61 REF_getField com/github/hashzhang/basetest/User.age:I
InnerClasses:
  //声明 MethodHandles.Lookup 为 final，加快调用性能，这样调用 BootstrapMethods 里面的方法可以实现近似于直接调用的性能 
  public static final #67= #63 of #65;    // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles

From here, we can see that in fact toString() calls java.lang.runtime.ObjectMethodsthe bootstap()method of . Its core code is:ObjectMethods.java

public static Object bootstrap(MethodHandles.Lookup lookup, String methodName, TypeDescriptor type,
                                   Class<?> recordClass,
                                   String names,
                                   MethodHandle... getters) throws Throwable {
        MethodType methodType;
        if (type instanceof MethodType)
            methodType = (MethodType) type;
        else {
            methodType = null;
            if (!MethodHandle.class.equals(type))
                throw new IllegalArgumentException(type.toString());
        }
        List<MethodHandle> getterList = List.of(getters);
        MethodHandle handle;
        //根据 method 名称，处理对应的逻辑，分别对应了 equals()，hashCode()，toString() 的实现
        switch (methodName) {
            case "equals":
                if (methodType != null && !methodType.equals(MethodType.methodType(boolean.class, recordClass, Object.class)))
                    throw new IllegalArgumentException("Bad method type: " + methodType);
                handle = makeEquals(recordClass, getterList);
                return methodType != null ? new ConstantCallSite(handle) : handle;
            case "hashCode":
                if (methodType != null && !methodType.equals(MethodType.methodType(int.class, recordClass)))
                    throw new IllegalArgumentException("Bad method type: " + methodType);
                handle = makeHashCode(recordClass, getterList);
                return methodType != null ? new ConstantCallSite(handle) : handle;
            case "toString":
                if (methodType != null && !methodType.equals(MethodType.methodType(String.class, recordClass)))
                    throw new IllegalArgumentException("Bad method type: " + methodType);
                List<String> nameList = "".equals(names) ? List.of() : List.of(names.split(";"));
                if (nameList.size() != getterList.size())
                    throw new IllegalArgumentException("Name list and accessor list do not match");
                handle = makeToString(recordClass, getterList, nameList);
                return methodType != null ? new ConstantCallSite(handle) : handle;
            default:
                throw new IllegalArgumentException(methodName);
        }
    }

Among them, the core implementation logic of the toString() method depends case "toString"on this branch. The core logic is makeToString(recordClass, getterList, nameList):

private static MethodHandle makeToString(Class<?> receiverClass,
                                            //所有的 getter 方法
                                            List<MethodHandle> getters,
                                            //所有的 field 名称
                                            List<String> names) {
    assert getters.size() == names.size();
    int[] invArgs = new int[getters.size()];
    Arrays.fill(invArgs, 0);
    MethodHandle[] filters = new MethodHandle[getters.size()];
    StringBuilder sb = new StringBuilder();
    //先拼接类名称[
    sb.append(receiverClass.getSimpleName()).append("[");
    for (int i=0; i<getters.size(); i++) {
        MethodHandle getter = getters.get(i); // (R)T
        MethodHandle stringify = stringifier(getter.type().returnType()); // (T)String
        MethodHandle stringifyThisField = MethodHandles.filterArguments(stringify, 0, getter);    // (R)String
        filters[i] = stringifyThisField;
        //之后拼接 field 名称=值
        sb.append(names.get(i)).append("=%s");
        if (i != getters.size() - 1)
            sb.append(", ");
    }
    sb.append(']');
    String formatString = sb.toString();
    MethodHandle formatter = MethodHandles.insertArguments(STRING_FORMAT, 0, formatString)
                                          .asCollector(String[].class, getters.size()); // (R*)String
    if (getters.size() == 0) {
        // Add back extra R
        formatter = MethodHandles.dropArguments(formatter, 0, receiverClass);
    }
    else {
        MethodHandle filtered = MethodHandles.filterArguments(formatter, 0, filters);
        formatter = MethodHandles.permuteArguments(filtered, MethodType.methodType(String.class, receiverClass), invArgs);
    }
    return formatter;
}

Similarly, the hashcode()implementation is:

private static MethodHandle makeHashCode(Class<?> receiverClass,
                                            List<MethodHandle> getters) {
    MethodHandle accumulator = MethodHandles.dropArguments(ZERO, 0, receiverClass); // (R)I

    // 对于每一个 field，找到对应的 hashcode 方法，取 哈希值，最后组合在一起
    for (MethodHandle getter : getters) {
        MethodHandle hasher = hasher(getter.type().returnType()); // (T)I
        MethodHandle hashThisField = MethodHandles.filterArguments(hasher, 0, getter);    // (R)I
        MethodHandle combineHashes = MethodHandles.filterArguments(HASH_COMBINER, 0, accumulator, hashThisField); // (RR)I
        accumulator = MethodHandles.permuteArguments(combineHashes, accumulator.type(), 0, 0); // adapt (R)I to (RR)I
    }

    return accumulator;
}

Similarly, the equals()implementation is:

private static MethodHandle makeEquals(Class<?> receiverClass,
                                          List<MethodHandle> getters) {
        MethodType rr = MethodType.methodType(boolean.class, receiverClass, receiverClass);
        MethodType ro = MethodType.methodType(boolean.class, receiverClass, Object.class);
        MethodHandle instanceFalse = MethodHandles.dropArguments(FALSE, 0, receiverClass, Object.class); // (RO)Z
        MethodHandle instanceTrue = MethodHandles.dropArguments(TRUE, 0, receiverClass, Object.class); // (RO)Z
        MethodHandle isSameObject = OBJECT_EQ.asType(ro); // (RO)Z
        MethodHandle isInstance = MethodHandles.dropArguments(CLASS_IS_INSTANCE.bindTo(receiverClass), 0, receiverClass); // (RO)Z
        MethodHandle accumulator = MethodHandles.dropArguments(TRUE, 0, receiverClass, receiverClass); // (RR)Z
        //对比两个对象的每个 field 的 getter 获取的值是否一样，对于引用类型通过 Objects.equals 方法，对于原始类型直接通过 == 
        for (MethodHandle getter : getters) {
            MethodHandle equalator = equalator(getter.type().returnType()); // (TT)Z
            MethodHandle thisFieldEqual = MethodHandles.filterArguments(equalator, 0, getter, getter); // (RR)Z
            accumulator = MethodHandles.guardWithTest(thisFieldEqual, accumulator, instanceFalse.asType(rr));
        }

        return MethodHandles.guardWithTest(isSameObject,
                                           instanceTrue,
                                           MethodHandles.guardWithTest(isInstance, accumulator.asType(ro), instanceFalse));
    }

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