Java Records实际上是通过相似的类声明来节省内存还是它们更像语法糖? [英] Do Java Records actually save memory over a similar class declaration or are they more like syntactic sugar?

问题描述

我希望Java 14记录实际上比类似的数据类使用更少的内存.

它们还是使用相同的内存?

解决方案

要添加到@lugiorgi >和我可以分析字节码的类似显着差异在于toString，equals和hashcode的实现.

一方面，现有的类中覆盖了Object类API的

public class City {
    private final Integer id;
    private final String name;
    // all-args, toString, getters, equals, and hashcode
}

产生如下的字节码

 public java.lang.String toString();
    Code:
       0: aload_0
       1: getfield      #7                  // Field id:Ljava/lang/Integer;
       4: aload_0
       5: getfield      #13                 // Field name:Ljava/lang/String;
       8: invokedynamic #17,  0             // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/Integer;Ljava/lang/String;)Ljava/lang/String;
      13: areturn

  public boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: if_acmpne     7
       5: iconst_1
       6: ireturn
       7: aload_1
       8: ifnull        22
      11: aload_0
      12: invokevirtual #21                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
      15: aload_1
      16: invokevirtual #21                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
      19: if_acmpeq     24
      22: iconst_0
      23: ireturn
      24: aload_1
      25: checkcast     #8                  // class edu/forty/bits/records/equals/City
      28: astore_2
      29: aload_0
      30: getfield      #7                  // Field id:Ljava/lang/Integer;
      33: aload_2
      34: getfield      #7                  // Field id:Ljava/lang/Integer;
      37: invokevirtual #25                 // Method java/lang/Integer.equals:(Ljava/lang/Object;)Z
      40: ifne          45
      43: iconst_0
      44: ireturn
      45: aload_0
      46: getfield      #13                 // Field name:Ljava/lang/String;
      49: aload_2
      50: getfield      #13                 // Field name:Ljava/lang/String;
      53: invokevirtual #31                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      56: ireturn

  public int hashCode();
    Code:
       0: aload_0
       1: getfield      #7                  // Field id:Ljava/lang/Integer;
       4: invokevirtual #34                 // Method java/lang/Integer.hashCode:()I
       7: istore_1
       8: bipush        31
      10: iload_1
      11: imul
      12: aload_0
      13: getfield      #13                 // Field name:Ljava/lang/String;
      16: invokevirtual #38                 // Method java/lang/String.hashCode:()I
      19: iadd
      20: istore_1
      21: iload_1
      22: ireturn

---

另一方面，相同的记录表示形式

record CityRecord(Integer id, String name) {}

产生的字节码少

 public java.lang.String toString();
    Code:
       0: aload_0
       1: invokedynamic #19,  0             // InvokeDynamic #0:toString:(Ledu/forty/bits/records/equals/CityRecord;)Ljava/lang/String;
       6: areturn

  public final int hashCode();
    Code:
       0: aload_0
       1: invokedynamic #23,  0             // InvokeDynamic #0:hashCode:(Ledu/forty/bits/records/equals/CityRecord;)I
       6: ireturn

  public final boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: invokedynamic #27,  0             // InvokeDynamic #0:equals:(Ledu/forty/bits/records/equals/CityRecord;Ljava/lang/Object;)Z
       7: ireturn

注意:就生成的访问器和构造函数字节代码而言，我可以观察到，它们对于表示形式都是相似的，因此也从此处的数据中排除.

I’m hoping that Java 14 records actually use less memory than a similar data class.

Do they or is the memory using the same?

解决方案

To add to the basic analysis performed by @lugiorgi and a similar noticeable difference that I could come up with analyzing the byte code, is in the implementation of toString, equals and hashcode.

On one hand, the existing class with overridden Object class APIs looking like

public class City {
    private final Integer id;
    private final String name;
    // all-args, toString, getters, equals, and hashcode
}

produces the byte code as following

 public java.lang.String toString();
    Code:
       0: aload_0
       1: getfield      #7                  // Field id:Ljava/lang/Integer;
       4: aload_0
       5: getfield      #13                 // Field name:Ljava/lang/String;
       8: invokedynamic #17,  0             // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/Integer;Ljava/lang/String;)Ljava/lang/String;
      13: areturn

  public boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: if_acmpne     7
       5: iconst_1
       6: ireturn
       7: aload_1
       8: ifnull        22
      11: aload_0
      12: invokevirtual #21                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
      15: aload_1
      16: invokevirtual #21                 // Method java/lang/Object.getClass:()Ljava/lang/Class;
      19: if_acmpeq     24
      22: iconst_0
      23: ireturn
      24: aload_1
      25: checkcast     #8                  // class edu/forty/bits/records/equals/City
      28: astore_2
      29: aload_0
      30: getfield      #7                  // Field id:Ljava/lang/Integer;
      33: aload_2
      34: getfield      #7                  // Field id:Ljava/lang/Integer;
      37: invokevirtual #25                 // Method java/lang/Integer.equals:(Ljava/lang/Object;)Z
      40: ifne          45
      43: iconst_0
      44: ireturn
      45: aload_0
      46: getfield      #13                 // Field name:Ljava/lang/String;
      49: aload_2
      50: getfield      #13                 // Field name:Ljava/lang/String;
      53: invokevirtual #31                 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      56: ireturn

  public int hashCode();
    Code:
       0: aload_0
       1: getfield      #7                  // Field id:Ljava/lang/Integer;
       4: invokevirtual #34                 // Method java/lang/Integer.hashCode:()I
       7: istore_1
       8: bipush        31
      10: iload_1
      11: imul
      12: aload_0
      13: getfield      #13                 // Field name:Ljava/lang/String;
      16: invokevirtual #38                 // Method java/lang/String.hashCode:()I
      19: iadd
      20: istore_1
      21: iload_1
      22: ireturn

---

On the other hand the record representation for the same

record CityRecord(Integer id, String name) {}

produces the bytecode as less as

 public java.lang.String toString();
    Code:
       0: aload_0
       1: invokedynamic #19,  0             // InvokeDynamic #0:toString:(Ledu/forty/bits/records/equals/CityRecord;)Ljava/lang/String;
       6: areturn

  public final int hashCode();
    Code:
       0: aload_0
       1: invokedynamic #23,  0             // InvokeDynamic #0:hashCode:(Ledu/forty/bits/records/equals/CityRecord;)I
       6: ireturn

  public final boolean equals(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: invokedynamic #27,  0             // InvokeDynamic #0:equals:(Ledu/forty/bits/records/equals/CityRecord;Ljava/lang/Object;)Z
       7: ireturn

Note: To what I could observe on the accessors and constructor byte code generated, they are alike for both the representation and hence excluded from the data here as well.

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